From the Super Mario Wiki
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If Donkey Kong and Diddy get too close to an AckStack, the Acks lean back and fall with their mouths open. Sometimes, AckStacks take up a large amount of space when collapsing and may knock the Kongs into an abyss. Donkey Kong can dodge an AckStack by jumping back, which causes every Ack to land separately, with their fangs stuck in the floor. These Acks are vulnerable to jump attacks. Unlike normal Acks, the Kongs can defeat them with a single hit. The Kong Roll is ineffective against AckStacks, but the Kongs are not injured from attempting this maneuver.