Context: MC is giving advice about sword practice to a young knight.
It seems that MC practiced JP sword art "kendo" back when he was still in his original world.
もっとわきをしめて。インパクトの瞬間に力を集中させる。
My translation: "Put more force to the armpit. Concentrate your power at the moment of impact"
If my TL is wrong please give me your English translation. I don't know much about kendo.