Functional flexibility: being able to achieve a range of motion that your muscles drive you into, and can get you out of, (not as pictured). Passive stretching to reach an extreme range of motion causes problems on your joints, tendons, and ligaments because you’re using external means (absent of muscle contractions) to do so. Since you didn’t utilize any muscle to reach that ROM, your body has to lever off of a joint to get you out of it. Like gravity just dumping your body (and your joints) wherever it wants to and then you have to figure out how to get out of that position because your body (muscles) didn’t actually do anything to get you there.
The fact of the matter is that your muscles are designed to lengthen (stretch) and then shorten (contract) and continue through this reciprocal function to produce movement. When you hold a stretch position you disrupt your muscles elasticity and they lose their ability to recoil (shorten/contract). So when we train clients to exercise correctly and recognize what a muscle contraction feels like, they use the contraction to move their body into a range for a few seconds and then they use the same muscles, differently, to move out of that range, and repeat for reps. With this intention, they’re building strength in one chain of muscle, while the opposing chain is stretching.
This allows the body to reach a safe range of motion without strain because when the muscle contraction disappears, you know you’ve reached a range of motion your body isn’t ready for. Never mind the stretch, remember if one muscle is contracting another is stretching. So focus on the contraction and the stretch will come along for the ride.