This is probably the singlest most important skill to master in your quest to get fast offroad.
Power is transmitted to the rear wheel from the engine, through the clutch. In any given gear, you can control it with both. You can in fact start thinking that you have one single control: the throttle-clutch - it is very rare that you will just operate the throttle or just operate the clutch in the woods.
On dirt, there is limited grip. When you want to acelerate, you need very fine clutch-throttle control, to get maximum acceleration. Likewise, to keep going on sand or mud or other loose surfaces.
On top of that, there are manouvers that require special clutch-throttle control, like wheelies. In a wheelie, if the front comes up too much, you should pull in the clutch a little more rather than roll off the throttle (while of course, using the rear brake).
To keep a 2 stroke in the power band, again, you use more clutch than throttle - even more so since they have no engine-braking.
The more power the engine puts out for the same throttle opening, the more you have to use the clutch to smooth out that power. In time, the smoother you get with the throttle, the less you need to use the clutch.
Watch a few motocross starts to get a feel for this. If you simply dump the clutch and twist the throttle, the bike will wheelie away from you, or not go anywhere. What you need is a very fine control of both levers, to deliver smooth usable power to the rear.
At the corner exit, since the bike is leaning already and the grip is reduced, again throttle-clutch feathering will get you out the fastest. If you get too much power, the rear wheel will slip out and maybe come around. If not enough, you get out of balance.
Feathering the throttle throughout the corner will smooth out everything. Jerking the rear wheel in the middle of a corner is a good way to get off-balance and clip a tree or a stump. Feathering the clutch throughout the turn is the key to a smooth, fast corner.
Again, clutch-throttle is important when getting the front wheel up over a log. You don't need a full wheelie, just to lighten the front wheel or even lift it a bit.
I got a KTM 990 SMT (see Upgrading the KTM 690 Enduro for 990 SMT) - a litre V-Twin which really does not like to lug along at low revs - it is so twichy that you can only ride it in slow traffic with fine throttle-clutch control.