In fall of 20 to 30 ft, the belayer has about 1300 ms from fall to peak. The point is the time to react and how to precisely synchronise his jump with the peak.
As USNavy says, you can define the peak as an even shorter period.
Try standing on a flat surface in belay stance and as quickly as possible, jump as high as possible. You might raise your CG by a couple of inches. That is, by a dynamic belay jump, you might be able to reduce the energy of the climber's fall by perhaps 1% or so ... if you time your jump precisely to the millisecond.
Belayers are better off forgetting about jumping to give an active dynamic belay, and locking off the rope correctly and focussing on not tripping or hitting the wall if on an indirect belay.