You'll get more responses if you post this in the Genera Care area:
http://www.redearslider.com/forum/viewforum.php?f=5I'm just going on what I have read here and other places. I'm a very new RES caretaker.
Depending on size, sex of the turtles, and sexual maturity it is either a mating ritual or a dominance one. If you search the forum for "fluttering" or "shaking" you will see many posts on this. As a part of the mating ritual, a male will nip at a female and flutter his claws in her face.
Apparently, turtles will also do this to show dominance.
If a female is involved, you'll want to read up on what is involved with getting allowing her to lay eggs properly, because otherwise, there are health risks.
If both are males, you may need to separate them.