[kevincox]

The fuel appears to be how much effort the compiler spends to find optimizations. If you give it no fuel it just spits out the simplest code it can generate. If you give it fuel it will "spend" that to search for more optimal code.

Since the simplest code didn't exhibit the bug and more optimized code did they were able to bisect the fuel value to find the particular unit of fuel that caused the incorrect optimization to be run. Then they could simply diff the two runs since they were incredibly similar (ideally just that one incorrect optimization differs) which allowed them to quickly narrow down on the root cause.