As tension is added, the wheel is put on the floor and the rim is pushed down hard with both hands. This is repeated at increments so that the action is even across the entire rim. The wheel is flipped and the process is repeated. The wheel is removed from the jig and de-stressed in this way four or five times as the tension is built up. If this is not done (some wheel builders don't bother) the wheel will de-stress during initial use, causing loosening of the spokes.