how do you pin the compressor housing on a hx40. what is the reason for this

 

You would drill and tap the side of the comp cover and use a hex set screw. You can add a small notch in the backing plate to hold the screw as well.
it is typically done to keep the cover from rotating under higher boost (usually twins) and blowing off the IC boot.
Some folks have bent/broke the snap ring and actually blown the comp cover right off!

Smearing a little tar or gasket maker on the edge of the backing plate works well enough for most. just be sure everything is CLEAN and scuff it with some scotchbrite first.