Trainer Feature
Trainer Features are various skills that your Trainer will unlock as they level up. If you wish to use a Trainer Feature during a Pokemon battle, most of these must be used during your Trainer Action step. However, there are two properties that change when you can use a Trainer Feature,
Static and Interrupt
Static Features are always active and do not consume any Trainer Actions.
For Interrupt consider the following Feature,
Give it Your All: Trainer Feature
Prerequisites: Pokemon Trainer
One Time Use Only
Effect: Target one of your Pokemon’s Moves. It is a Critical Hit.
Interrupt
You would use this feature after your Pokemon used their move, causing it to become a Critical Hit. Your Trainer could have used a Trainer Action prior to this, but the Action for the next turn would be consumed. However, you could use another Interrupt on that next turn, because Interrupts only consume the following turn's Trainer Action.
If you ever have a question about when you can use a Trainer Feature, PM a crew member.