On 747-100 and -200 aircraft, each reverse thrust lever is mounted on the front side of it's respective thrust lever with the hinge above the reverse lever's end knob. If you should happen to grab the reverse thrust lever without having the forward thrust lever all the way back, the action of lifting the reverse thrust lever will force the forward thrust lever all the way back. When you lift reverse thrust lever to an almost-level position from it previous position parallel to the forward thrust lever, the buckets come out. That in itself provides considerable braking, and it's common to stop there. Continuing to lift and then bring back the reverse thrust levers spools up the engines. You need to be careful doing this. Coming back too far too fast might cause a compressor stall, especially on an already marginal engine. Compressor stalls on a Pratt and Whitney engine are no big deal. On a General Electric engine they are far more serious. As I remember, there was a lock-out that prevented an engine from moving into reverse thrust from other than idle thrust, but I can't remember how it worked, but I believe it required not just idle thrust but the aircraft also being on the ground (squat switches) and landing flap deployment.