Hi YP
It's the Hadd training advice - basically he makes you do a "sub-lactate threshold" runs of 10m at 80% to 83% max - the idea is that if you can keep the same pace without the heart rate going north of 83%, ie zero drift, for 10 miles, then that's your mara pace (conversely if you run faster than that / experience drift before 10m, you'll bonk on a mara).
Over time you can push the sustainable 10 mile sub-lactate run up in HR terms eg 81% to 84%. Can you run an even 10 miles with no increase in HR. Yes? Move up to 82% to 85% etc...the reasoning is that with no drift, you can't be above LT. With drift, you're producing lactate, so if you go on too long at that pace you'll bonk.
I picked 80% and stuck to it - in fact in Frankfurt I stayed below 80% until mile 13, then did a negative split. It works!