How does the force of ones karma determine where the mind stream goes?
This question is very similar to another: Does Mahayana Buddhism discourage attaining enlightenment under the Gautama Buddha Sasana? In my answer to that question I explained how the bodhisatva who post-pones enlightenment developed. 


  How does "the force of karma" determine where the mind stream goes after a physical death?


Many different answers have been proposed and adopted by Buddhists of different schools at different times to try to account for how karma determines where the mind stream goes after death. But no one really knows. We say that this is what happens, but a satisfactory explanation of the process is entirely lacking. We sometimes use analogies to try to explain, but they don't really tell us anything about the reality of this process. 

The bodhisatva choosing their rebirth is simply an article of faith.