Elliot has unlocked a new ability! By which I mean he realized he could do that thing he just did and could probably do for a while but didn't know about.
Many pointed out that Elliot's hips wouldn't have changed in a way that resulted in the situation in the last panel, and have claimed that, if anything, his pants would have been held up more securely. To THAT I said, once again, that the transformation was a process, and that a lot of subtle movements during that process could have resulted in that situation.
Besides, the next comic is counting on it!