Greetings!
To answer your question, I will go into a bit into what "flow drills" and "sensitivity drills" are. This is because most drills in those categories, as mentioned, develop both attributes.
Flow drills, loosely defined, are cyclic sequences of movements that can be seen as a strategic attack pattern. The partners can do counter for counter until one is hit, or the cycle continues. YOu can change rhythm or strikes as to hit your partner. They tend to make the practitioners comfortable in such close quarter spaces.
Higher evolutions of these drills deal with higher degrees of pressure, power and force, so very strong fundamentals are vital to get to this point without collapsing to the pressure. MAny that do not go into this evolution of the exercises, when encountered with real pressure, tend to collapse, use extra force and loose their structure. This breaks the pattern.
Thus the flow drills have to be executed with maximal structure and verification that each movement holds maximal pressure with minimal effort.
Sensitivity drills have a similar execution, exccept that patterns are not set. It depends on intention, reaction, intuition and timing, among others. These parameters will direct the action-reaction-counter relations, thus it might seem that the practitioners flow from one movement to the other, the more spontaneous nature of the exercise produces more realistic stimuli.
Now comes the cool thing.
Higher levels of sensitivity develop perception-proaction-action-reaction-counter relations!! This means that intentions can be "felt" or "percieved" before or instantaneously preceeding physical action. Call it experience, or propioceptive conditioning, yet this phenomena occurs.
This permits a level of proficiency that allows easier control of attackers.
Note that the development of such skills requieres very prescise fundamentals training and strict application of these basic skills to the flow drills. Also note that Chi sao can be taught as small chunks of flow drills, with specific stimuli as triggers.
How where all of you taught Chi Sao? (I know it's an individual answer for each.)
Note that most of the statements I've written are easily demonstrable by qualified instructors.
This is just the tip of the iceberg.
Enjoy!
Juan M. Mercado
Teaching Combat Sciences since 1995