Passive stretching
Passive stretching should only be done at the end of training sessions. The aim of passive stretching is to relax the muscle and help it recover.
Contrary to popular belief, you shouldn't carry out passive stretches during training. This is even discouraged as it increases the risk of tears and muscle injury. So ultimately it does more harm than good.
Passive stretching is very much a post-training routine.
How to do a passive stretch? You should hold the stretch for 20 to 30 seconds. You're really looking to relax the muscle in order to put light tension back into it. You should definitely avoid feeling any pain as this is an inefficient way to stretch that increases the risk of muscle injury.
Ideally you want to stretch the muscle when it's cold rather than when it's hot. This is a new technique that has been approved by many high-level sports men and women. You need to wait until the muscle has cooled down before stretching, normally around 1-1.5 hours.