During that short period of time while the engine is flashing from 3000rpm to 5500rpm, the engine's rotating assy is actually absorbing some of its torque, which in-turn momentarily reduces the amount of torque applied to the converter's shell. That added flash time gives the car more time to transfer weight onto the back tires before all the engine's torque gets applied the converter's shell.

When you launch closer to the converter's stall, less engine power gets absorbed by it's rotating assy, so less time to transfer weight.

Grant