Two possibilities
It is a promise that the other person does not leave you much choice, but squeezes it into a narrow space. Make your decision immediately, make it final, use your preference means to distract us too much. This head will be given one way or another, the head will go away, whatever you do is futile. If you spit down it means a beard, if you spit up it means a mustache.
Sometimes it is conditional, some work. When you are given something, it is said to think about the end while the work you have to do is put before you. The end of the work is either problematic or troubled, or fortunate, that is, two is probable. Somebody is rocking you in the gallows tearing away from life. There is a good reward if you succeed and a lot of rewards, but if you lose there is an end that is not good for you.
Is it viable, does it have a place in real life, if it is said, it is and is possible. Some things work like that. It has two options with two options. The decision is up to you. You either accept it or part your ways. If you win, you will be rewarded. If you lose you will be forced to choose whether it is forty mules or forty lines. Both mean death like death.