このような自由な身体の運びを「ヘン・変」と言います、そして、変じると同時に自然に相手に出来た隙をすかさず技を出すことを「オウ、応」と言います、。「変・応」は二つのものではなく、　両者一体のものであります。

解釈：変じると同時に自然に相手に出来た隙をすかさず技で反応する。

“You need to be able to move your body to a safe place to defend against his attacks, being able to move your body freely to get to this place is called Hen (Change), this causes his openings to quite naturally present themselves and by being able to change freely and take advantage of them, this is called Ou (Application?), Hen and Ou are actually one and should not be thought of as two separate things.”

Interpretation :
as you naturally flow with the opponent’s movements, it creates openings, which you must recognize and act upon. (Technique wise, a good example would be an arm bar from bottom when someone tries to pass your guard or you just escaped their side control and their armpit is open. #find a link)

This reminds me of Rafael Lovato Jr.’s Flow-Pressure-Finish (#add link)
Also the idea of triggers or affordance. (#add links)

Context :
Kyuzo Mifune (三船 久蔵 Mifune Kyūzō, April 21, 1883 – January 27, 1965)

In the video they mention that Mifune has trained for 54 years, and his biography states that he started at age 13, so the footage must be from 1950 when he was 67 y/o.