well the test is done firstly on the ground so weight keeps the spring in place ,then on ramps ditto then the tester places a block/jack underneath the vehicle and always places the block under the rear suspension arms and lifts and because of the position of the block keeps the weight over the springs so its never a problem. i have had cut down springs that seem loose for a long time and the car has visited four different test centres and not one has ever picked up on it or placed the jack in a different place. if you are worried then drill some small holes and wire it in to the body.
