Use this video to add a cool down period after your workouts when they don't include one.
Using low resistance, gradually slow your legs down to progressively bring your heart rate down. Then follow with stretches. Drink plenty of fluids.
5-10 minutes easy spin for most workouts
10-20 minutes for longer workouts
A cool down helps return your body to its pre exercise state and will aid recovery and adaptation processes.
A progressive cool down will help flush out metabolic waste products such as lactate from your muscles.
A cool down will help re-circulate the blood back up from your lower extremities to your upper body. During cool down the gentle muscle contractions in your legs will help push blood back up. This will prevent blood pooling in your legs and will reduce the chances you will feel dizzy, nauseous or eventually faint after a very strenuous workout.
A cool down allows you to mentally wind down after a hard workout
Stretch out your hamstrings, quads, calf muscles, hips, and back. See this article as an example of post-workout stretches.