At the moment, it is not possible to fast-forward or rewind a class. This is to make sure you train safely and complete the warm up and cool downs. It also affects the tracking feature embedded into all classes.
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You are able to pause a class and resume it from where you left off.  

You can see the full breakdown of all the moves included in the workout prior to taking the class, just tap the 'Class Breakdown' under "Class Details" before you start the class.