Originally posted by rexxer
It's simple physics... momentum.
Conservation of momentum:
mass1 * velocity1 = mass2 * velocity2
When you depress the clutch you disconnect some of that mass from the system, therefore the velocity of the system instantaneously increases.
That logic is flawed. The mass you remove is still spinning. It's momentum is not magically transferred to the flywheel upon clutch engagement.
The rpms rise because power is being generated in the engine (fuel igniting) without any load.
The power either comes from the ECU dumping fuel for a split second until the intake pressure is relieved (maybe to prevent lean combustion that would increase the EGT's and output of nitrous oxides) or a slow reaction between the release of the gas pedal and the closure of the TB.