Sort of an overhand, and his body rotates a lot, but also his head whips to the side right as he's bringing the ball around.
Some video here:
https://www.mlb.com/rays/video/ian-seym ... -blue-jays
Found an interview with him on Fangraphs
Laurila: Your delivery has been described as unique. How would you describe it?
Seymour: “I’ve kind of just always thrown a little ugly. Nothing I do really looks clean or pretty. For instance, my head ends up behind me every single time I throw a pitch. That just feels normal for me. It feels weird if I try to look at where I’m throwing.”
Laurila: When a young pitcher has a funky delivery, coaches will often try to train him into something more conventional. Has that happened with you?
Seymour: “No. They’ve said that as long as I’m throwing my pitches in the zone, there’s no need to change my mechanics.”
Laurila: Do you agree with scouting reports that have described your delivery as “very rotational with plenty of crossfire”?
Seymour: “Honestly, I don’t even really know how to describe it. I just get up there and try to throw it as hard as I can — right through the catcher — and it ends up looking like that.”
Laurila: What about your footwork? You kind of step back on the rubber.
Seymour: “I definitely have a lot going on. I think my glove arm will end up over my head at one point. The tapping thing I do [on the rubber] is something I’ve done for a long time. I just can’t sit still, really. I’m pretty hyperactive, and I guess that’s just how it presents itself when I’m pitching.”
Laurila: Would it be accurate to say that your effectiveness is based on a combination of deception and pitch movement?
Seymour: “Yeah. I’m not throwing 100 [mph] or anything. What I do have — according to our system — is outlying metrics on a lot of my pitches. And I’ve heard people say, ‘I’ve never seen someone’s body move like that when they throw,’ so I guess that does add a little bit of an uncomfortable nature to it.”
