Instead

If an ability says "When you would do X, do Y instead," you have to be able to do Y in order to avoid doing X.

Example

The item Shoulder-Mounted Parasol's ability is: "When you would gain any amount of light fatigue, remove 1 light fatigue from your cooldown track instead." You must be able to "remove 1 light fatigue from your cooldown track", in order to avoid gaining light fatigue.

You must have light fatigue in your cooldown track, to be able to use the Shoulder-Mounted Parasol to avoid gaining light fatigue.