Taijiquan's "wu bu" can be translated as "five steps" which might mean "five stepping directions," but equally it could mean "five stances" or more broadly "five movement ideas." As with Taiji's "ba fa" (eight methods), the ideas are simple and generic: they do not refer to specific techniques or shapes.
The easiest way to think of the five "steps" is as 5 directions - forwards, backwards, left, right and centre. But it isn't quite that easy - almost, but not quite. Translated, the actual Chinese terms used are more dynamic than mere points on a compass rose.
Jin means to advance. This describes an action that moves forwards towards an opponent.
Tui means to retreat. This means moving backwards in response to an oncoming attack.
Zuo gu is usually translated something like "look left." Literally it means "left turn around and look at." Evidently we are being instructed to turn to look at something or someone that is to our left.
Yuo pan means "right look." The word "pan" also has connotations of looking expectantly, so it again clearly indicates turning to face an oncoming attacker. The attack is imminent.
Many Taijiquan commentators have described the need to try to orientate your whole self in any given direction, so that you can move with connected whole-body power. "When you move, your whole body moves." So you should not just move to the left or right, you should also turn to face that new direction. Yang Banhou talked about "the three forwards" (both feet, hands and eyes turning towards your attacker). Similarly, the Xingyi classics talk of "three points on a line" or, in other words, three points that must be focussed in the same direction - the foot, the hand and the nose.
(As a brief circuitous diversion, I'd here like to add that Baguazhang's creator Dong Haiquan also taught his students to continually orientate themselves towards their foes. One of Bagua's main solo exercises, circle walking, involves just that - moving around a post or tree while continuously twisting to face it, usually training both of your hands on it at the same time.)
Now, back to Taiji.
The final "step" of our "five steps" is zhong ding. "Zhong" means "centre" or "middle." "Ding" means "calm" or "stable." Zhong ding then refers to a state of centred stability, or as it is often rendered "central equilibrium."
Chen Pan Ling explains this as any time we have our weight evenly distributed on both legs. By extension, we could then see moving to the left as any time we take our weight (sideways) onto our left leg, turning our torso to face the new direction as we go. Moving right would be seen as any time we shift on to our right leg, again turning to face that direction. Now, if an enemy was directly in front of us, we might take our weight onto our front leg to advance, or on to our rear leg to retreat. Simple. Whether you are moving forwards or backwards or from side to side, each time your weight passes through the centre, it passes through centred stability (zhong ding).
A second way of thinking about the "wu bu" (five steps), is to think of them more literally as steps. You might need to step in any of the four directions, or you can stand and deal with attacks on the spot (zhong ding).
It is said in the classics, "to go left, you must have first gone right." Zheng Manqing warns us to never turn (pivot) or step with a weighted leg. So if we wish to step, we must have first emptied the leg we wish to step with. But then we will fill this leg again as we perform our chosen technique, turning to face the new direction.
When dealing with whether to step or not to step, our fifth option, zhong ding, means staying more or less where you are while dealing with a foe. Your feet do not have to remain rooted to the floor in this instance - you just need to counterbalance each of the attacker's attacks and defenses. When dealing with physical conflict, maintaining "central equilibrium" is a highly dynamic undertaking. You need to maintain centred stability in relation to your attacker, in order to maintain balance within yourself.
So in response to an attack to your left, you should momentarily "yield the left side" as instructed in the classics. To an attack to your right, "your right side should melt away." If an attack is coming straight towards your face, you might wish to drop below the strike, lowering your stance, and execute the technique of peng (an upwards diverting action). The effect is like a lever or seesaw - you drop down and execute peng to knock your opponent upwards. The easiest way to drop down low is while maintaining a 50/50 weight distribution ratio between your legs, which brings us back to Chen Pan Ling's explanation of zhong ding as an even distribution of weight.
Incidentally, it is even easier to sink low if you open out your stance as you do. Here I like to think of my spine sliding straight down a fireman's pole, so that I remain vertically balanced too (this upright centrality is called "zhong zheng.")
With regard to advancing and retreating, the classics tell us that "when attacked we must get ever further away, but when our opponents retreat we should crowd them all the more." So we do not meet force head on: this would be meeting Yang (force) with Yang (more force) - a decidedly unbalanced ratio. Neither should we try to meet Yin (weakness or retreat) with Yin, other than in the sense that we must remain responsive to changing events. We have to know when and how to strike back - to be Yang to the opponent's Yin. We must to be ready to utilise any gaps that appear in the opponent's defenses; to seize the opportunity to conclude the encounter. We do not want the enemy to get away, only to come back and kill us or our comrades another day. Or in civilian life, we might decide to apprehend the attacker before they can escape to harm anyone else.
A final consideration is that Taijiquan is a circular art. We might raise a hand to meet an oncoming strike (using the fighting method of peng), but then transform the movement along a curved path into ji (to strike forwards), lie (to chop sideways), lu to divert the attack subtly off course, etc.
We can see the five directions in the same circular way. We might start a technique against an attack coming from our side, stepping sideways and turning to face the oncoming attacker. But is this then moving sideways or forwards? You might wish to see it as both. You may start by moving sideways but as you turn, the new direction gradually becomes forwards.
Furthermore, when executing the eight methods, we frequently perform them in combination from the outset. For example, it is quite possible to divert an oncoming attack obliquely, perhaps combining peng (diverting upwards) with lu (diverting sideways); or to divert an attack downwards as well as sideways (combining lu and an). Similarly, we frequently need to step in a diagonal direction, combining a sideways stepping direction with an advancing or retreating one, to make an advancing or retreating sidestep.
In conclusion, it can be seen that the 5 steps of Taijiquan are not specific techniques or prescribed methods, but a generic conceptual framework for understanding movement. As with everything else in Taijiquan, you are required to understand the idea and execute it intelligently. While some might protest that the art is not easy, I think it is fair to say that any perceived complexity actually stems from the fact that it is almost too simple.
What is Taijiquan? Taijiquan is up, down, forwards and back. Taijiquan is advance, retreat, centre, turn left, turn right. Everything flows together endlessly like a mighty river. Now you do it...