If you are wearing Gear with the Shield keyword, you may spend 1 Focus to activate it whenever a character within Close-quarters of you is about to lose Endurance.
Generally speaking, the Endurance you are about to lose is the result of a roll. You may activate the Gear after the result of that roll is known, but before losing the Endurance.
The Gear specifies it will only reduce Endurance loss from specific sources, that still counts.