The post EQ switch changes the signal routing in the unit so that after the preamp the signal goes to the EQ first and then the compressor. By EQing the signal before it is compressed you can alter how the compressor affects the signal. If a certain frequency is boosted then the compressor will squash the signal more when that frequency is present in the signal i.e. the action of the compressor is not just dependent upon the signal level but also the frequency content too.
The best example to use is a drum beat. If the low end is boosted then the compressor will really squash the signal every time the bass drum sounds, resulting in a 'pumping' effect on the cymbals. This is common to hear in dance music, where it is important that the kick drum has a real punch and pushes through the mix. Click here for a more detailed explanation.
With most signals, you will find that you want to EQ after compressing but there is no harm in trying it the other way round - at the end of the day, you are looking for what sounds best to you.