Bgct83 hit the nail on the head. The S90's are the Arsenal bindings, except with an upgraded ankle strap - the same strap that's used on the 390's.
So what you get is a blend of a stiffer, more responsive highback and the softer flex of a super cushioned ankle strap.
If you've got any other questions feel free to hit us up on email - ridercouncil at romesnowboards dot com.
- Rome SDS