Whats the dealio? (sub-threshold means below lactate threshold or anaerobic threshold)
The deal is that you can do relatively high volume of HM-M-paced (if you are a faster runner) work with slightly higher quality and slightly less recovery need. It is basically tempo work (tempo= sub threshold), but just split by shorter rests (50-60s max). Reps can be from 60s-10min and rest then 20s-60s. Typically it is done as a combo of workouts through the week touching M-pace with longer reps and HM-effort with shorter. Lactate is used to control the intensity and not pace (but runners learn what pace will give the right lactate)
It is just a way of training approx 80% easy and 20% fast, where the 20% is for the most sub-threshold to threshold in terms of lactate but at a spectre of paces through various rep lengths.
One easy way of applying is to exchange the tempo with intervals at the same effort or lactate, but the pace might increase slightly