Hi Audi,
I think this is clearly a case of 用勁 being used in an ordinary sense of "exertion of force" rather than 勁 as a proprietary taiji term of art. I think the point being made in the Pressing Kick instruction is that it's not necessary to exert much force for the technique to be effective, especially if the opponent has lost his root. Just as a gentle An or Ji or whatever hand or arm movement can send an opponent away when applied at the right moment, the same holds true when applying the sole of the foot. When I was first taught the form, I was told that for the Separate Feet kicks, where the striking surface is the upper outer side of the foot, the objective is a sort of stinging slap to the sensitive flank, or the armpit, of an opponent. Then I was taught that the dengjiao kicks were meant to just make contact and kind of gently send the opponent off. The whole notion of severing the opponent's root and send him off (提放 tífàng) is neatly described in another part of the classic commentary in Brennan's Shiyongfa translation:
練十三勢要用柔法,然後功成就生出柔中含藏內勁,呼吸者,蓋吸能提得人起,能使敵後足離地,再呼氣力從脊內發出全身之勁放得人遠出,呼吸靈通,身法然後才能靈活無滯。
When practicing the solo set, it should be done with softness. After you have worked at it for a long time, you will be generating internal power hidden within the softness. “Breathe” means that when you inhale you can lift [提] the opponent, making his rear foot leave the ground, then when you exhale, power comes from the spine, issuing with the power of the whole body, and sends [放] the opponent far away. By mastering breathing, your body’s techniques will then be quick and crisp.
Take care,
Louis