First you'll want to create a new auxiliary buss that we will use for the parallel buss.
Once it has been assigned you can then send, say your snare track to the buss.
Assign an effects send to the corresponding buss, then set at (0) and engage the pre-fader mode on the track fader.
Ensure that Delay Compensation is activated in ProTools or you will experience phasing issues between the original track and the parallel buss.
Now you can add your choice of compressor to the buss.
Bring up the snare track and the newly created parallel buss and listen to the differences.
What can be achieved here are typically two things:
1) The ability to push the level of the parallel compression though a dense mix
2) Create an additive tone to the original track(s)
I like to hit the parallel compressor pretty hard. I use anywhere from a 3:1 to 6:1 ratio. Depending upon the musical content, I may assign a faster or slower attack, but usually pretty fast. The release is set to react very quickly so that the snap of the snare is instantaneous and doesn't hang around long.
I also like to add EQ and multiple compressors in serial all to the same buss. In this instance I am using the buss as both a parallel compressor and a "tone control."
It is also perfectly normal to use more than one snare track in a mix. For instance, you may have a top snare, bottom snare and sample tracks. Once you have established the composite tone among these tracks, you may realize that not all those tracks have to be bussed. Perhaps only one needs to be assigned to the parallel buss. Maybe you wish to bring out the ring of the top snare a bit more in the verses. The sky's the limit, really.