I remember reading this article on pressure with the bottom arm. Here it states the vaulter must be fully inverted before applying bottom arm pressure. I can see some similarities with what Agapit is describing with the pressure used immediately upon take-off. Agapit, your thoughts?
http://pvei.com/fusion/readarticle.php?article_id=14
Pole Thrust
by Mark Hannay
During the flyaway phase of the vault, a "summation of forces" occurs and is the resultant of the cumulative forces of the pole's rotation to a vertical position, the vaulter's rotation around the handgrip, and the unbending elastic energy of the vaulting pole. It has been scientifically established that the total sum of energy at the end of the vault was derived by these three factors each converting and storing approximately 33.33% of the lifting energy. The accumulated energy is responsible for the vaulter's vertical lift and vertical velocity during the flyaway portion of the vault.
It is important to note that both the vaulter's position to the vaulting pole and the timing of getting into the ideal position prior to the flyaway will determine how much of the energy is available or lost. Assuming that the vaulter is properly inverted, in a position close to the pole, and aligned with the pole he/she will be able to utilize nearly all of the available forces that the summation will provide.
An advanced technique for maximizing the effect of the pole's thrust and flyaway velocity is to have the pole's unbending forces returned by the vaulter firming his/her bottom grip hand and arm. The stored energy of the unbending pole will be transferred to the vaulter through a shorter lever by a stiffer portion of the vaulting pole. For example: A vaulter is using a 460 cm. (15'1.5") 200 pound design vaulting pole. His/her top hand grip height is 14'6" with a 13'0" bottom hand grip results from an 18" hand spread. If the unbending pole's thrust is applied to the vaulter through the top hand, the amount of the unbending force will be applied through a 14'6" lever and an amount near the capacity of the 200 pound design of the pole. However, if the vaulter focuses on clamping down on the bottom grip hand and arm during the pole's thrust, the pole's energy will be returned through a 13'0" lever that will unbend similar to a pole that is of a 227 pound design.
This technique takes advantage of two factors. First, vaulting poles have a measurable amount of resistance to bend (stiffness) at any point along the shaft of the pole. That amount varies from pole model to model, but an average amount would be approximately 1.5 pounds per inch of hand movement along the pole. If the top grip hand is move up, the pole's stiffness would decrease approximately 1.5 pounds per inch. If the top grip hand was moved down, the pole's stiffness would increase approximately 1.5 pounds per inch. Therefore, if the vaulter's hand grip spread is a distance of 18", the apparent stiffness of the pole unbending through the bottom hand is a significant 27 pounds stiffer than the apparent stiffness of the pole returning through the top grip hand.
Second, short bent levers unbend faster than long bent levers. If the unbending pole is directed to return its unbending force through the bottom grip hand and arm, the force will be returned quicker and increase the impulse of force on the vaulter.
Finally, by focusing the unbending pole's energy to return through the bottom hand and arm grip, the available forces will be returned in a quicker and more powerful manner. If the vaulter is in the ideal inverted position at the proper time and he/she is focusing the pole's thrust through the bottom grip, it will increase the flyaway velocity of the vaulter and potentially increase the height that the vaulter will vault.
Please note: Caution should be used when teaching and/or using this technique. If the vaulter prematurely focuses on firming the grip of the bottom hand and arm, the vaulter may fail to get completely inverted. Too firm of a bottom hand grip during the swing-up phase preceding the inversion will decrease the ability of the vaulter to become completely inverted. It is not until the vaulter is fully inverted that the bottom grip firming technique should be applied. Furthermore, this technique is not recommended for vaulters with limited vaulting experience. Advanced vaulters who consistently vault 18" or more over their top hand grip height should be able to effectively use this technique.
wo xi huan cheng gan tiao.