It works by showing you a pulse on the string, then you tune the string, to slow the pulse down, and stop it completely, to be in perfect tuneThe problem is, if the string is way out of tune, you will not see the pulse at all

If you need to do non-standard tuning, you'll have to use the strobe against a fretted string; for Drop D tuning, for example, you'll need to fret the low E at 2 and use E6 for comparison.