I have seen that video before, I think. It really didn't clarify much (for me) related to swinging a bat. Which extensor muscles? How and when should they be in action?
"Extensor action is exclusively the steady effort to straighten the bent Right Arm. This stretches but does move the Left Arm and produces a structural rigidity that is a strong deterrent to collapse under the stresses of Acceleration and Impact."
The pressure applied by pressure point number three to help keep the left arm straight. I like to feel pressure point number 1 applying pressure to the left thumb throughout the swing as well, this has the affect of straightening the left arm and eliminating slack from the arm swing. I also feel as though it helps to keep the hands at their address height in the takeaway, important for drawing a straight plane line and thus maintaining initial posture.