Depends. It's a muscle, similar affects as a muscle seems to apply. You can have a combination of variables at play. Flexibility, strength, endurance, etc.
They all have a dynamic that affects the overall. Someone who is flexible may still get crazy tight when stimulated. Like almost snap your fingers tight, but relaxed is not tight. Someone who lacks flexibility can be tight even when relaxed and may not be much tighter when stimulated. Anxiety, stress, etc. also can cause tightness and can lead to pain which can be relieved by relaxing. Thats a cycle that can be a bitch to overcome, especially if the partner doesn't care.
I see it as a complex muscle group with many different triggers...fun to play with as it's very "cause and effect" based [emoji317]
I find loosening and tightening is based on how you are being stimulated