Took me a while to realise the hammer is bouncing off the... target?.. and back in the space between two frames - it looks like he's swinging the hammer, then it stops moving and he headbutts it. Ahh .gif stroboscopic effects.
It looks like there's a plank or something above the round thing I guess he's supposed to aim for, and the hammer bounced off the end of the plank. He's got a terrible swing! LOL
Phil
“The biggest problem people have is they don’t think they’re supposed to have problems.” - Hayes Barnard