This is typically only required on cars with direct injection. With DI, fuel does not wash over the intake valves, so there can be carbon build up. This process is pretty common on the Mazda, Audi/VW, BMW DI platforms. Port injection cars typically don't need to do this.
You need to have some kind of sealed adapter so you can blast with the media gun but still have a shopvac suck up the media. The crank needs to be turned so that the valves are closed.
When I had mine done, we used an old radiator hose, drilled a hole for the blaster and used a shopvac on the other end.
I don't know about you guys, but I did some Vdyno 4th gear plots before and after and didn't get any gains at all. Car drove the same, compression test before/after the same, vaccum readings the same. I run methanol injection, EGR delete and an OCC, so perhaps that may have helped with reducing carbon build up. Some of the pics of Audi/VW's look nasty.
Ex:
Valve before - The worst one I had.
Ex:
Valve after
Vdyno plot
Red - Before
Blue - After