Behind the girth means only about 8-10 inches back of the girth. This will control the horses hind end and is sometimes called leg yielding. The same type of pressure is used as at the girth. These leg aids are used progressively to obtain obedience. Leg yielding works by rewarding the horse with a release of aid by lightening pressure or ceasing to act or resist.
Resistance prevents or opposes a lateral displacement of the horses hindquarters.
In other words it keeps a horse from swinging his haunches to avoid the work asked of him.