I have just gotten a Hit-Air vest. I tried one at a show, so I was familiar with the design. However, this vest is slightly different.
Instead of the nylon strap I tried, which attached to the stirrup bars and clipped into the vest, it appears to require a grab-strap on your saddle (attaching to the D-rings, not the stirrup bars). Then you'd have to loop the attachment around the strap, then clip it to the vest.
However, when I tried it on at the show, I was told specifically by the Hit-Air person not to attach the vest to the D-rings in any way, because there's a chance of breakage and thus the vest not deploying. Also, couldn't the strap break?