The direct out works very much like miking the amp but without the limitations of having a mic sitting in front of the amp. All you need is to run a balanced XLR cable over to the mixing board for the PA and set up for a mic level signal. As you turn up the amp, the direct output gets louder, so adjust the mix to get as much guitar you need in the PA. 

The direct out is taken from the speaker side of the power amp which adds to the richness of tone because the low dampening factor of the amp allows the speaker to react with the circuit. The direct out also has a little bit of low pass filtering to help tame harshness in the overdrive and makes it sound closer to what you hear from the amp's speaker.

You can disconnect the speaker and use just the direct output, but you lose a little bit of richness and the output voltage increases without the load. A good compromise to keep the on-stage volume down is to run the amp's master volume at a lower level and add more guitar into the house PA mix.

Even if you aren't used to miking your amp, why not try sending the direct out to the mixer and letting your sound engineer have some extra control over the guitar mix. You might see a subtle improvement in your band's sound.