How to Do It
Do a long set to failure, then let the weight stretch that pumped muscle for 30 seconds as you try not to cry. Intra-set stretching is not only one of the best methods of enhancing mobility under loading, but also devastatingly effective when it comes to muscular hypertrophy secondary to metabolic stress. Intra-set stretching techniques are simple to program and execute. Here's an example:
Machine Flyes with Intra-Set Stretching
After doing a set to failure in an ultra-high rep range, you're simply going to let the load stretch you out passively for around 30 seconds, or as long as you can tolerate the pain. Aggressively stretching a fully-pumped muscle is the perfect mechanism for growth. You increase overall muscular tension while also maximizing the cell swelling response for maximal muscle damage.
The key here is to take your set to absolute mechanical failure – meaning you can't complete another rep even with maximal effort – before you let the weight stretch you out. When it's time to hold that stretch, you better be ready for some mental warfare.
Be careful not to compensate your joint positioning during the stretch phase and put your body at an increased risk of injury. You're going to be in some pain here, but don't deviate your position, especially under loading. Use clean form and good technique.