Friction Zone in Harri's words

The partial application of power to the rear wheel caused by "slipping" the clutch.

That amount of clutch usage that allows any amount between 1% and 99% of the power the engine is producing to continue tot he rear wheel.
A common problem that causes riders to "lose" the friction zone is that they use the wrong part of their fingers on the clutch lever. Although it seems like it would be easier, and thus better, to have the lever between the knuckles of the fingers, you'll actually get a better feel for the lever, and thus better control of the lever, if you use your fingertips. Look at your left glove. If the wear pattern on the fingers (from the lever) is close to the palm, you're gripping the lever wrong.
Because of the confusion about engaged/disengaged (or clutch/de-clutched in Britain), I use the terms "relax your grip" and "increase your grip". This makes it understandable. I say "relax" rather than "release" because release would mean to let the lever go, and that of course gets you out of the friction zone.
Harris