In the yang styles of yoga, the teacher will allow a significant amount of time at the end of the class to cool the body down. Again, in the yin practice this is not necessary. We never warmed the body up, but we still want to find a way back to neutrality, to balance. Any of the beginning asanas could work well at the end of a class, but the pose most often done is the reclining twist. This asana allows the body to fully relax and release. It is one of the most yin-like asanas of all.
The twist in the spine can be directly higher or lower to relieve whatever area was most worked in the practice. Moving the knees higher toward the armpit brings the twist more up the spine by curving the spine forward. Pointing the knees straight away straightens the spine, allowing the twist to be even along its length. Moving the knees downward arches the spine slightly, bringing the emphasis in the twist to the lumbar/sacrum.
Of course, twisting the spine can be done in many orientations. You can do it sitting up as well as lying down. And twisting the spine is not the only way to end your practice; but twisting does restore equilibrium to the nervous system and gets a lot of the residual kinks out of the system.