Changing the interval in a workout to stop at MPA Not Time

hi i was just wondering how to change my workout screen in my app on android too stop each interval at a certain fatigue and not the time… iv seen a utube clip of this happening and if you are low in power the timer increases time and if your hitting the right power or above the timer speeds up, how do i change the settings to do this for my workouts , thanks.

Variable “smart” intervals occur as long as you select a Smart labeled workout from the library (or suggested list) and run that workout on the Xert mobile app or Connect IQ app on a Garmin.
If you want to design your own workouts with this functionality see this article –