Boulder Trap (Yellow)

“When this defends” is an event-based trigger. The power of the attack is checked at the time this resolves and becomes a defending card (i.e. defends the attack). If the trigger condition is met only after this resolves, then this does not trigger.

This effect is not optional. If the opponent controls at least one equipment, you must put a -1 counter on one of them.

You may put a -1 counter on an equipment, even if it doesn’t have the defense property, or it would bring its defense value below zero (when actually calculated, negative defense values become 0).