Like most things, breathing is the most important part of stretching. That said, it is also one of the most difficult parts of stretching.
When you’re doing strength training, you know that during the eccentric part of the movement (the lengthening of the muscle) you inhale, and during the concentric part (the shortening of the muscle) you exhale. It’s the same idea with stretching. When you are strength training the objective is generally to lift as heavy as you can during the concentric movement and the exhalation helps with that. When you’re working to increase flexibility the objective is in the lengthening of the muscle, so that is when the exhale is necessary.
A general rule to have in mind when you’re stretching is; you inhale when you’re still, and you exhale when you’re moving. Whether it’s a static stretch, or a dynamic stretch if you breath correctly you will go further and have greater gains.
The easiest form of stretching to understand this concept is PNF (proprioceptive neuromuscular facilitation) With PNF stretching you contract your muscles (not moving, so inhale) and then you relax your muscles (moving deeper into the stretch, so exhale)
The next time you stretch try it out. Focus on your breathing! It will help you relax more, and it will give you something to focus on other than the pain so it’ll go by fast too!