These bindings are amazing. The soft heelcup and tension system put them ahead of everything else out there. Once you ride them you won't go back to rigid plastic injection molded bindings. I'm so surprised they haven't caught on quicker, could be the price point(or the fact they don't give them away to every pro). I got rid of my Rome 390s and Union Force they couldn't hold a candle to the relays. Definately one of those tech advancements that makes riding easier, more expressive, and more comfortable.
No it doesn't diminish heelside turns, in fact I think it transfers energy more effectively especially if you ride correctly (feet/ankles)and aren't one of those people who initiates heelside turns by railing on your highbacks. I have Rome targas on one of my boards and I'm going to replace all of my setups with the different Relay models Pro/XLT/Thruster.
The Relay system is a traditional strap system. Nothing like Flow binders that I can think of. The big difference is the soft heelcup. You can literally squeeze it together. So it's supposed to give you more flexibility when in the air etc. Yet performs like a regular binding. I put a pair on my splitboard for backcountry riding. Not so much that I am doing spinny flippy tricks, but because the Relay Pros were nice and light. Less weight on the way up equals more turns going down. Performance wise, they are great. No issues with response and they are more comfortable than a binding with a rigid cup. Ground breaking? Not so sure, maybe the park rates will say more. It works fine and it is a quality binder.
Just get a pair. I guarantee if anything you will be pissed you didn't spend the money to try them before. I ride alot of park and they are awesome primarily because they are so comfortable and secondarily because they allow more movement of your ankles etc without giving up board control. You won't look back.