One of the hallmarks of many FMA systems is “flow” drills: any drill, symmetric or asymmetric where two practitioners move smoothly (and sometimes with astonishing speed) from one position to another, offering offence, counter-offence and re-counters. Many of these drills start off with a prescribed set of movements, but all build towards a student developing “flow”: performing the above, but with no set series of motions. It is not sparring, nor is it like boxing focus mitt work; I am challenged to think of a good corollary in other systems (which is not to say none exist!).
A common criticism of these drills is that they lack “realism”. Why spend valuable training time doing something that does not mimic how people — trained or untrained — actually fight?
Tuhon Leslie Buck shares his thoughts on some of the problems with flow drills, and offers perspectives on how to use them effectively in your FMA practice: http://store.kaligear.com/blog/putting-the-function-back-into-your-flow-drills/