We are not talking about uncontrolled slip and runaway rpm. We are talking slowing the application of clamp pressure within a clutch that has plenty of capacity to lock up after slipping...not a clutch that slips because it's operating at it's limits.
Depending on the amount of rpm change over time, it is possible to slip a clutch for a longer period of time while experiencing fewer actual revolutions of slip. Here's a comparison of slip revolutions for equal time periods
Setup #1- An un-damped launch from 6500 dropping to 4000 .5 seconds later at clutch lockup represents about 22 revolutions of full pressure clutch slip.
Setup #2- A controlled release launch from 4500 maintaining 4500 rpm thru .5 seconds represents less than 19 revolutions of slip with gradually increasing clutch pressure.
Setup #2 has less actual revolutions of clutch slip, but it also has the advantage of 500 less rpm loss. Spending it's launch operating closer to it's torque peak, it reaches the shift point quicker.
