The goal in HIIT is to push as hard as possible during the work phase and recover as much as possible during the recovery phase.
For example starting with a 30 sec work phase and 30 sec rest phase try to work as hard as possible during those 30 “work” seconds then take deep breaths and rest during the 30 seconds “off”
In class everyone is not the same level! So during HIIT don’t worry about the time that trainer set! just make sure your form is good and work as hard as YOU can
When fatigue sets in slow down a bit but try not to stop. When your energy returns just increase the pace again.
By this way you will get the real result from HIIT class.