Kurt, does the replicas upper receiver draw back, or do you mean the little T lever (like in a M4 rifle) pulls back? Some replicas don't have the same feature as the original guns.
In a real Mauser C96 you flip the safety to Fire and draw back the internal slide, but the upper receiver stays stationary (and will lock open if unloaded, otherwise it feeds a round). The hammer is now cocked and the safety may be put to safe now.
It will slide back slightly (under a quarter inch) and will do so when fired but remains for the most part stationary with the inner slide shooting back and over the hammer. The hammer does not self cock mechanically but gets slapped back from the slide ramming back cocking it.
The C96 is very odd, and it's to expensive to shoot ($2+ a bullet) but it looks so cool.
My C96