How does it work? Is it just the luck of the draw whenever you steal someones rank?

Rank stealing is usually done when both players have a rank thats within 1000 elo of eachother.

If im rank 4500, and my opponent is 4250, and I win, we are most likely going to trade ranks

I'm guessing thats not certain though. Because I saw zomfier today (ranked 1k) fight some alt and lose and his rank skyrocket to 22k. He got a little pissed

ELO. The little numbers beside your rank on the Chat. Say I'm rank 100. My ELO will be at 1700. And someone that has rank 150 with an ELO of 1702 fights me, they win and they get my rank because the ELO difference. Sometimes they will take your rank just because they have a better ELO even if they lose.