When activating this card, select 1 unused Monster Zone on the field.[1]
You cannot activate this card during the Damage Step.[1]
This card's effect that returns it and a monster that was Special Summoned to the selected zone to the hand starts a Chain. (It does not target, and will also activate during the Damage Step.)[1]
This card and the Special Summoned monster both return to the hand simultaneously.[1]
Q&A Rulings
Q: When activating "Boompoline!!", can I select an unused Extra Monster Zone?
A: Yes, you can. After "Boompoline!!" resolves, if an Effect Monster is Special Summoned to the selected Extra Monster Zone, the effect of "Boompoline!!" will activate.[2]
Q: I activate "Boompoline!!" and select an opponent's unused Main Monster Zone. If my opponent chains "Back to the Front" and Special Summons a monster to the selected zone, does the effect of "Boompoline!!" activate?
A: If a monster is Special Summoned to the selected zone by an effect that was chained to the activation of "Boompoline!!", the effect of "Boompoline!!" does not activate.[3]
Q: "Boompoline!!" is face-up in my Spell & Trap Zone, and my opponent Special Summoned an "Evilswarm Thunderbird" to the Monster Zone that was selected. At this time, if the effect of "Boompoline!!" activates, but my opponent chains the effect of "Evilswarm Thunderbird" and banishes it, what happens?
A: In this scenario, since "Evilswarm Thunderbird" is already banished when the effect of "Boompoline!!" resolves, and is not in the selected Monster Zone, the effect of "Boompoline!!" does not apply. ("Boompoline!!" remains face-up in the Spell & Trap Zone.)[4][5]