I didn't think that his "bronco buster" showoff was specifically a "tantalizing" of Ennis, but it sure showed many other things: that Jack and Ennis could talk to each other, that Ennis could make fun of Jack's 'career' and Jack could reply in fun; show the 'rodeo f***up' that Ennis's father had described, fall on his ass, and still laugh. Ennis is essentially asking him to shatter a myth he got from his father, and Jack does that. It's part of Ennis's growth with Jack. Recall that (in the film) that's the first time we see Ennis happy and smiling. If you want "tantalizing", I think Jack sitting naked in the tent the second night is a better example. It sure worked. ;-)
btb