Hey Francis, the pitch is set by the oscillation of the air column which is determined by the tension of the lips plus air compression, so both I suppose – The Sound Equation: Sound = Flow + Tension
Watch Compression Depression since you are an Ultimate Level member, it is explained in detail there.
The best psychology to eliminate over-blowing is SHAPE change without pinching the middle of the lips. Be sure to maintain the feeling of the “AaahOooh” and the BCH!
It will all become very clear as you watch through the course.