Once a pawn has crossed the line, he has to stay. There's no way back.
7 seconds sound clip from the Mystery of Chess Boxing movie soundboard.
You can hear this line at 00:45:49 in the DVD version of the movie.
Quote context
[...]
- Mister, I just hope you meet him and then you'll teach him not to avoid people!
- Out!
- I know this. We're gonna find out!
- Once a pawn has crossed the line, he has to stay. There's no way back.
- Teacher? Something I must say. I don't want to, but I must.
- Go on.
- Well, in fact you're already learning.
[...]