Attach with two very strong straps, and I guess you have to be prepared to cut some slits into your seat unit to make the lower straps stay put securely. They also come with a pair of velcro straps which can be riveted to the hip area on the seat unit too, and have foam shims which can be inserted into the pads for fine tuning. Totally solid fitting.
In conjunction with an IR Reggie backrest my boat feels so good now!!