Flexibility and mobility are often thought to be synonymous terms to explain how you move your body from point A to point B. This is a false misconception and something we need to understand to really grasp how to improve both our mobility and flexibility training.
The word flexibility is defined as, "the capability of being easily bent without breaking". In fitness, this is known as passive range of motion, where you are moved into a range of motion with help from an external force (manually, band, holding onto a squat rack, etc.).
Mobility, on the other hand, is the ability to move throughout a range of motion actively through your own voluntary muscular contractions. When we stretch, we often claim to be training mobility. In reality, passive stretching leads more towards improvements in overall flexibility.
This is where Passive Range Holds play a key role in minimizing the threshold between our mobility and flexibility limits.
By irradiating (building total body tension), and isometrically contracting once in our passive range of motion, we're teaching our nervous system how to stabilize and actively control a foreign range that we can't actively control. While this might result in cramping initially, this is one of the best ways to better improve your mobility to try to match your current flexibility!