It's a little complicated. It's for people that don't use their cars (or at least spend more on the gautrain than they do on their petrol) as much as they do the gautrain, such as myself. Instead of the cash back applying to your fuel spend, it will apply to your gautrain spend. And this cash back amount will depend on how many DQ points you get in that particular month (max is 750 or 1000, can't remember). Halve the points, and that's the max cash back you can receive. So for example if I got 600 DQ points for last month, and spent R2000 on the Gautrain, I will get R300 cash back for the month.
Hope I explained clear enough, nothing about insurance, medical aid, etc is simple!