80/20 Rule -- Is This a Meme?

What's the group consensus on the 80/20 rule? For those not familiar, it's a training regimen which suggests that the athlete train their body at a slow, slow rate for the duration of their exorcise (80%), while exerting themselves maximally for the remaining 20% of the exorcise. Has anyone tried this practice for an event? Cat V, Triathlon, etc.?

*My girlfriend got me a book on this subject and instead of returning the book (because it sounds like BS) I figured I'd at least ask others for advice first.

Thanks