I recently played a bullet game, in which I thought I was in checkmate, but then realized I had one legal move, this then repeated again and again 6 times until ultimately I was in checkmate. I share this anecdote as an explanation for the seemingly arbitrary parameters.
To reiterate, one player (black or white) puts the other player in check. That player has only one legal move (moving the king, blocking the check, or capturing the attacking piece) Then the other player puts them in the same circumstance of one legal move. Player one does not need to be put in check or have their moves limited, only in the sense that no positions may be repeated.
The final check of the sequence is checkmate.