Most of these posts are easy to understand by the general martial arts public, but "sensitivity drills" involve a more specialized training that may be harder to understand from verbal descriptions. In a nutshell, sensitivity drills are two-person drills where most reactions are determined by contact feel. Well known sensitivity drills include the "push hands" of T'ai Chi Chuan, "lubud/hubud" of Filipino systems, and "chi sao" (sticking/clinging hands) of Wing Chun lineage. However, it is easily arguable that any grappling and wrestling training is dominantly sensitivity based.
While the purpose of ground based sensitivity training, feeling for position and responses of your opponent, are generally clear, the standing drills mentioned are a bit more abstract. That is, it's not easy for the non-practitioner to understand; who is attacking or defending? Why not just strike etc.?
Addressing chi sao specifically (and remember this is a personal perspective that might differ from the traditional), the drill is based on the assumption that a confrontation can easily get "messy," that is, when there is an interaction between opponents that leads to contact of the limbs, possibly with no clear direct strike available. In such a scenario, chi sao provides the ability to feel what to defend in an efficient way, and how to feel or create an opening in your opponent's position at the same time.
While many kickboxing based folks deride chi sao as too refined, it is entirely relevant combatively. While a confrontation could certainly be ended with strikes and no sticking, it’s not at all unusual for things to get sticky owing to the natural chaos that can result from things moving and changing up faster than ones perception. As with the closely related trapping hands, one doesn’t “try” to stick, but it is very easy to find yourself in a chi sao related position. For example, if someone threw a haymaker and you “cornered” (biu sao da for the terminology-concerned) and they tried to block or muffle your incoming strike, you would be for an instant in “double inside” position. For another example, if you parried a jab as you hit up the midline (a time-hit which I like to refer to as a “split” for ease), and the opponent tried to parry or muffle your incoming strike, you would, for an instant be in “right handed” position. Also, something I observe is that it’s not unusual for someone throwing a strike to advance and not entirely withdraw the hand, creating an unintended sticking point.
Two ways I like to train students when showing the relevance of chi sao, are 1) playing the slob, i.e. after learning proper rolling, I play the untrained part of someone over-reacting to a strike in a continuous flow of messiness from one to another to give a naturalistic feel to the drill. I periodically counterstrike so they don’t get too complacent. I also emphasize direct striking when there is really nothing to stick to, and 2) Have them defend against a flow of strikes with the normal parries and covers, but stick only when I give them that attached energy.
This might not be the easiest to visualize, but worth playing with.
Bruce Lee "threw out" chi sao, who cares? He threw out many things that I'm not good enough to throw out. My understanding is that he thought that if someone was good enough to stick with him, he was good enough to beat him, and who was that going to be?