Probably because either:
A. The handle is so flimsy he needs the chain brake for extra support.
B. The chain brake fell off and he's holding it there so it doesn't hit the chain.
Nope, not either of those problems at all.
Because it's such a POS, they couldn't get anyone with real chainsaw experience to touch the D'mned thing. So they hired a model to put on the gloves and make like they were running it. Poor model didn't know any better, probably thought it was really cool to have TWO handles at the top to hang on to, just in case one broke.
Any resemblance to "live action" is probably computer enhanced graphics, because they couldn't get it to start when they went to the photo shoot.