+ How do you know your partner is behind you?
+ How do you know he won't take your gun after he uses the taser on you?
+ How do you know it was a taser?
+ Are you confident enough in those answers to miss your daughter's next birthday?
So he knows his taser has the stopping power of a nerf gun at that point. The corollary is that he knows the taser in the hands of the victim is also out of range, also has the stopping power of a nerf gun. After this is known, because it is known, he drops his taser and draws his pistol, which does have the range.