My understanding is:
If a fund has shareholder equity of 100 and also borrows 100 it has 200 to invest.
If it earns a 10% return on the 200, it receives 20. If it then pays interest of 5 on its borrowings it is left with 15 for its shareholders - a return of 15% on the 100 of equity.
Repeat the calculation with 900 of borrowings and 100 of equity and you get 55% return on the equity after paying 45 of interest.
If, instead, it earns -10% on 1000 then its shareholders are wiped out and there isn't enough to pay the interest on the borrowings.