, lensman wrote

The use of !timer1.Enabled relies upon a quirk of the compiler.  It is not guaranteed to work 100% of the time on all systems.  Yes, it does today, but tomorrow may be another question.

It's guaranteed to work in any system which has valid boolean logic. I'd kind of worry about any that didn't.

Maybe you're thinking of some dev's occasional over reliance on assumptions about the specific numeric value of a boolean return which don't always hold true.