Why loaded stretching instead of just holding stretches longer?
Because strength through a range is what makes your body stop guarding it. Passive holds get you temporary range; loaded work through the same range is what keeps it.
What are nerve glides for?
For some people the limit is tension along the back of the leg rather than short muscle. Glides move things gently rather than stretching them. If pointing your foot changes how far you can fold, that's a hint this matters for you.
How heavy should the Jefferson curl weight be?
Light enough that you can control the entire descent for 5 seconds and never lose the sequence. For most people that's 2 to 8 kilos. This is a mobility exercise wearing a weight, not a lift.
I felt something sharp behind my knee.
Stop that session. A strong pull through the belly of the hamstring is normal; sharp pain behind the knee, or numbness and shooting sensations, means back off and get it checked before continuing.