Your opponent set up an automatic response line. You made the move he THOUGHT you would make and so his planned response to your move kicked in and its your move again.
Free players can only put in one line. It is of limited use... but if you solved for mate via forced moves, you can enter the moves and forget about that game. Or any other small forced sequence etc ... it helps speed things along. I use it once in a while.
Thank You. That helps a lot, although I don't see where I would use it. You learn something new everyday!
You would use it in an online game with days/move delay and a forced move series. That is all it is good for. Or you could pay up and set up 2 or 3 or whatever lines.
It just saves time. Imagine a cut and dried series that trades 3 pieces through a series of captures etc. You set up the series. Your opponent logs in and moves, and it moves for you. He moves again, and it moves for you again... this can shave days off an already long game in certain positions. It is more useful when one or both players log in infrequently.
What does it mean in Online chess when you get the message "Your move has triggered a conditional move. Your move again?" - instead of the usual message "Your move has been submitted..."