The idea in each case is to ramp VO2 up to max as quickly as possible and thereafter choose a protocol that enable you to maintain VO2max as long as possible ie: by delaying the depletion of W'. In the intermittent protocol there from what I've seen in a couple of studies you should be able to maintain VO2max for a good 15-20min or so. The idea is to stress the aerobic energy system maximally at the lowest possible mechanical stress on the legs.
