Well I didn't studied Syn's style that much but from what I noticed (and feel free to correct me if I'm wrong):
- In fast solos he sometimes do chromatic scale runs
- His vibrato is wide, like you have to move your wrist a lot to sound aggressive
- He uses his Sustainiac pickup a lot to replicate the sound of an amp feedback
- A lot of harmonized guitars with Zacky
- A lot of divebombs, especially in City of Evil
- In The Stage, a lot of pinch harmonics
- Even in its fastest and hardest solos, he never does a solo full of shred like scales he would play up and down. They are always quite melodic and alternate with harder and more shreddy parts, and always stays very melodic
That's a bit messy but that's what comes to my mind for my observations. And his style has evolved a lot between each album.