It is possible to say that there are two great tendencies in the practice of chi sao: one in which the focus is mainly on the use of the hands and is developed mainly static, and one in which the use of the displacements.
Although training chi is without displacements, or almost without them, can be useful in an initial phase of the learning, the fact is that to maintain this modality as main carries important dangers in being effective in the combat. In fact, once the steps in the chi are mastered, all (or practically all) techniques should be done with displacement.
The initial distance between practitioners during chi is a distance too long for a stroke thrown from it to be effective. For a blow to produce the shock necessary to achieve a k.o. it must produce a jolt, going through the opponent’s body. It is for that reason that to hit at a realistic distance in which, if we wished, we could cause real damage to the adversary we must advance towards him. So to strike correctly from the starting point we must take a step with the attack.
On the other hand when defending the attack of the adversary, although sometimes we can stop the adversary without going back, the truth is that if we get used to using only our arms as a defensive tool we run several dangers. On the one hand we use only one defensive resource, while if the defense is accompanied by a displacement and our hands fail we will have more possibilities to avoid the attack of the adversary. On the other hand and although we do not fail, if the opponent launches an attack with a large displacement, even if we avoid the blow with the use of the hands without moving, it is possible that we stay so close to the body of the attacker that our own movements are hampered by lack of space. Obviously the opposite error that we must avoid is to move too far and leave the minimum distance of chi sao, moving away more than necessary.
Finally the use of steps in chi sao, both to attack and to defend, allows us to work the angles. We always strive to ensure that our anatomical center line (the one that divides the body into two symmetrical halves) coincides with the real central line (the plane that joins our spine with that of the adversary), and that is not so for our adversary. This gives us an advantage over it, and can not be achieved without working the techniques with steps in the chi sao. Equally learning to avoid being dominated in the central line is also something that we achieve by training the chi with steps.
In short: training chi sao techniques without movement can be an appropriate initial (although not necessary) phase for the beginner. Training chi sao with great use of steps gives us a more useful practice to develop effective skills in real combat.
*Video is in spanish