Post is selectable on the aux mix(es) only, IIRC. This will make your main fader(s) cause the level in the aux mix(es) to be effected.
Okay, you want channel 1 to be sent to aux 1. Aux 1 is set to post. Channel 1 in aux 1 is set to the zero mark. Now, if you bring up the slider in the main mix, the level in the aux will also go up.
Likewise if the aux1 mix is set to Pre1 and you bring up the fader in the main mix, then the level in the aux will not go up. It will remain at the level you set it at in the aux mix.
There is not a setting called Post2 or Post1. Just Post. Post meaning after the main fader.
I hope that clears it up.