People learning kung fu must concentrate hard, or else. If you're not concentrating, then you can't stand firm.
10 seconds sound clip from the Executioners From Shaolin (1977) movie soundboard.
You can hear this line at 00:39:36 in the DVD version of the movie.
Quote context
[...]
- Don't underestimate Tiger kung fu.
- But still, Pai Mei is not an ordinary kung fu fighter.
- Now don't start. I refuse to learn Crane style. You save it for our son.
- To depend on Tiger style, you'll have to practice ten years.
- Hands by your side. They mustn't move. Now, stand firmly.
- Ma, let me get off early today.
- Every day's the same. Stand as long as you can.
- Look at him, Hung Wen-ting, standing so still!
[...]