Ambassador Counteraction in Coup
If player 1 takes an action then Player 2 has 3 options.


They let the action go through
They can challenge the action and if they are correct then Player 1 loses a card, if they are wrong then Player 2 loses a card.
They can claim a blocking action and that leaves Player 1 with the first two choices that Player 2 had.


What the big difference is that if Player 2 claims a block then the decision and risk of losing a card shifts back to Player 1. Which in the case that Player 2 is correct it would mean Player 1 could lose a card for the challenge. So it puts more risk on Player 1 then Player 2.