here is a basic explanation for why to ride low intensity. as long as you are in the aerobic realm, you skew your body to believe it is resting when it is not. That is to say, if you're doing 150 watts of work at 120 BPM HR, your body will adapt quickly and you will be able to do the same workload at less, say 110 or 105BPM. In this way, you essentially raise the ceiling of where aerobic becomes anaerobic. Why is this important? Lactate threshold is VERY important.