The ischial strap's main purpose is to keep the splint bound securely to the superior part of the injured leg and should be placed as high as possible on the patient to capture as much of the hip as possible. The D-rings of the splint's ischial cushion should be on the top, while the ischial strap should be on the bottom; this allows for the strap to be "shimmied" up under the patient and tightened easily without significant patient movement.
Once the splint is in place and you are ready to place the ischial strap, have the patient hold the main body of the splint while you slide the strap under their bottom until the strap is almost midline. The strap may then be wrapped upwards over the patient's hip as high up as possible and cinched down with the D-rings on the main body of the splint. The chance that the splint will migrate during transport is reduced as more of the hip is "captured" by this strap, making it a critically important step.