The hex shank of the bit fits into a special chuck that only accepts such hex shanks. The front ring on the chuck will slide, an let you insert the bit, than snap back into place.
RPM's are varied by pressure on the trigger, until the bit meets resistance. At that point, the bit is 'hammered' in the direction of rotation. You will be able to see the bit turn, stop, turn in time to the hammering. The force of the hammering is not something you can control - the strength of each blow is set by the tools' design.
Here's the impact driver thread from around Christmas:
https://www.electrical-contractor.n...l&topic=0&Search=true#Post172107Ian A.
