They were sent to the north secretly to learn a new technique of kung fu, which they could use to suppress the rebels, especially Shaolin kung fu.
12 seconds sound clip from the Mysterious Footworks of Kung Fu movie soundboard.
You can hear this line at 01:22:08.615 in the Blu-ray version of the movie.
Quote context
[...]
- Three-Eyes Liu, you have a good heart and you are brave.
- I know about you from beggar man. He told me what happened to you.
- You're lucky to be alive. You fought with real experts. You might have died.
- Mister, you mean you know these men?
- Yes, he does. 20 years ago, he had a fight with 'em.
- A fight?
- Yeah!
- At that time, the Shrimp and the Crab were the champions, fighting for the Emperor.
- Many revolutionaries have died at their hands. They're dangerous men. So I...
- Since then, they have never been defeated.
- Master, why haven't ya done it?
- I can't. I'm too old. And my broken legs can no longer stand it. It's too late.
[...]