IN the old days, criminals were beheaded. They put their heads on a block of wood, and the executioner cut it off. There was a rule though. If the prisoner had a good friend who was convinced of his innocence, he could change places with the prisoner. The prisoner would have a week to prove his innocence. If the prisoner could not prove his innocence within a week, the friend lost his head. The friend puts his head on the block for the prisoner.

I have never used this phrase and I am a native English speaker.

Yes. To put one's head on the block (for someone or something) means to take great risks for someone or something; to go to a lot of trouble or difficulty for someone or something; to attempt to gain favor for someone or something. Examples: 1. I don't know why I should put my head on the block for Joan. What has she ever done for me? 2. Sally tried to get me to put in a good word about her with the boss. But the last time I put my head on the block for anyone, it all backfired, and when the person stuffed up, I looked like an idiot!