Originally Posted by klw
Looks like a curve to me. Do you have a GIF of him throwing a fastball? He seems to slow his body during the delivery more than you would for a slider but I would be curious to compare this to his fastball.
To be fair, I used an online place to create that GIF, so I can't even say if that is in full "real time". It seems close to it, but it may not be perfect.