Blood Flow Restriction Training
I recently came across a video by Trainerroad about this (you basically tie off your thigh to limit blood flow with the idea being that this simulates intense training (not enough blood, or rather oxygen, being delivered given the muscles' demands) and can help increase your Vo2 max.
I can't help but feel that this is an incredibly stupid thing to suggest (even with the usual "see your doctor", "talk to your coach" disclaimers). It's ******g dumb to cut off your blood flow, much less when doing intense exercise.