When a reserve flys a greenslip on an off day, those hours are added on top of reserve guarantee, plus, you get the off days back. So, fly two 20 hour 4 day greenslips, plus say two more 4 days on a regular reserve day.

70 hours guarantee + 40 hours GS pay = 110 hours pay for actually working 80 hours, plus you get 8 off days back.