You agree to buy A amount of gas for $X per unit (gallons, litres etc). When you want to, you withdraw from your reserves which have been prepaid. That's fixed.
So if gas prices other places are higher than X, say $3.60 while your X was $2, you're "saving" the difference. You may want to withdraw from your reserve at their stations.
If gas prices are lower than your X, say $1.60 obviously you're going to buy gas elsewhere and not withdraw from your $2/unit reserve.
You choose when you wish to withdraw.