So, I'll do a warm up of 5-10 minutes - or sometimes I'll do this at the end of a 45 minute zone 2 session.

Then I do a hard effort that I can just barely sustain for 4 minutes, then a 4 minute active recovery. I repeat that cycle at least 4 times. The trickiest part is going at an effort that is not too easy, but not so hard that it can't be sustained for the full 4 minutes.

I think there are studies that show that you gain the most VO2 max benefits when your intervals are from 3 to 8 minutes long with an equivalent recovery period.

As for modes, it can be about anything. I switch it up. I either run, do them on my trainer, bike outdoors, or do them on an assault bike. But you could row, xc ski, or I guess any other aerobic exercise that you can sustain at a steady state.