Subtractive EQing is useful when you're using low quality EQs which introduce less artifacting to the signal as compared to boosting. Cheap filters create combing when you boost em basically.

It works by cutting the inverse of the frequencies you want to boost, then pushing the gain back up again. So say for example you wish to boost the bass a bit, you'd put a rather dry low pass filter on the signal to deminish the high end a tad. The result will be a signal that has more bass in it than mid and trebble.

Perosnally I don't find this technique to really be of much use, because I usually end up ruining my sound in one area whilst trying to correct it in another. IMO Just use boost when you need to and drop the overall gain. Make sure you're using a decent EQ though... There are plenty of options (some free). If you can't tell the difference, then who cares!