Booby Traps

You can deploy it only in an opponent's sortie phase, after dispatch (in the second window or the final one), while they have crew in the mothership battle attacking you. Never in your own turn, in a watch or before dispatch. It is checked then, not again as it resolves: if every attacker has left by then, it still does what it can (802.12). Attackers stay in the battle until the end of the sortie phase, so in the final window it reaches the ones that survived the battle.

No battle is chosen: the mothership battle, as the card resolves. Every crew there takes it, your own defenders too. Plating takes it first, the damage stays, and a crew left with no hull is destroyed once all of them have taken it, before the battle is fought if it is still to come. A crew that has left the battle by then isn't hit (705.7).

While it is in your scrapyard, you may deploy it from there for this cost instead of its own, whenever you could deploy it from your hand. It goes into the crossfire, and the opponent may react. Deployed from your hand, it goes to the scrapyard as usual, so it can be used again this way.