How does one improve/work on active flexibility?

How can one improve their active flexibility?

 

A few general examples

 

1. Straddle and pike flexibility for straddle and v ups.
2. Raising your leg slowly into the air and holding it there, like those karate guys holding kicks.
3. Doing splits in handstand, or shoulderstand, or doing splits when hanging off a bar or rings (I don't know if this has even been done).

 

 

We can distinguish 2 phases in the long therms programming:
1- major part of the work I done in order to release tension into antagonist muscles while active strength that's nothing more than strength training it is held at minimum
2-passive stretch is held at minimum (maintenance) while strength is the major part of the training

Specifically,
Stretch courses address phase 1
Foundations address phase 2