In Quantum Leap, Sam's theory of time travel explicitly put forth that one could "travel within their lifetime," but it doesn't explain how he "jumps" into other people's bodies/existences in that time stream.
What's more, it's not exactly "them", but Sam "projecting" their appearance. This was made clear when Sam "leaped" into the body of a man with no legs, but was fully able to walk normally. Now, some type of physical connection must exist [as when he was in the body of a pregnant woman, and when she went into labor, he felt the pain]... but it's not exactly made clear how.
Is there some canonical source that gives some idea as to how this works?