Always noticed that, when idling, as you push the clutch in the revs go higher - very, very slightly, possibly only 100 or 200 rpm more at most - and stay at that level until you let the clutch out again. This is with the car not in gear.

Anybody know any reason for it doing this? What's the engine management doing?

(It's nothing like the initial higher idle RPM after you first start the engine, that's way higher)