Couchstretch.com may be about crossfit and mobility, but the name couchstretch.com comes from a mobility stretch called the couch stretch.
The couch stretch is one of the most useful stretches (in my opinion) as it helps to stretch out the quads, and, more importantly, helps to open up the hip area specifically the hip flexors. both of these are over used and, when tight can cause all sorts of issues during crossfit, and contribute to lumbar pain (back pain).
The Couch stretch is also easy to do, so there’s really no excuse for not doing it either. simply kneel on the floor with one leg forwards and the back leg bent at the knee with the foot up against the wall and the knee as close to the wall as possible, you should then feel a stretch on the front of the rear leg where your quads are. Squeeze your butt and keep your torso as upright as possible and you should start to feel the area around the hip of your rear leg start to stretch as well.
For more information on specific corrections to the couch stretch see here.