They don't affect the ECU per se.
When the standard valves dump, the air they dump is refed into the intake system, but on the turbo side of the AFM's - the ECU has already metered this air, and is feeding fuel to cope.
However, if you let the valves dump to atmosphere instead, and don't tell the ECU, then it puts too much fuel in. Then, you have unburnt fuel in the exhaust system, and if it's hot enough and you don't have a cat to get in the way, the fuel ignites for flames!