I seriously cannot find any good, thorough responses to this question, and I'm trying to help out a non-native friend.
Sit down implies motion. I understand that because of the preposition "down". Sit, too, can imply motion. The only difference I can find between the two is when you try to make "sit" transitive, it simply doesn't work. "He sat me DOWN" "he **sat me".
That being said. All of the following sentences (for me) are correct: "please sit"/"please sit down"; "why are you sitting?"/"why are you sitting down?"; "I can't reach this sitting"/"I can't reach this sitting down"… That last one could sound a little off just because it sounds like you are trying to reach a sitting of food or something, but that's about it. In all verbal tenses I still understand the two of them the same way. So is the only difference between "sit" and "sit down" the fact that "sit down" can be transitive and "sit" not?
If someone could give a more detailed response about the differences between the two, I'd appreciate it. I don't want to give a flimsy response to my friend.