Once the sheet is placed on top of the pad, shifting should not be an issue. (greenmountaindiapers.com)
The other additional buttons sit on top of the pad, between the bumpers and the triggers, slightly towards the centre. (techradar.com)
The method you can visualize this with is this... think of being on the Rotary Hip Machine with a leg over top of the pad, then bring your leg down and back. (fitstep.com)