u dun need to install half life, just the steam install with cs and it'll automatically install hl with cs for you.

as for the shield exploit, it has to do with a certain combination that makes the server thing the shield is blocking yet the pistol is out ready to shoot, basically, it looks like the shield isn't in front of you to everyone but it really is and you get to shoot back without taking damage, even a direct headshot wont kill you from the front since the server thinks the shield is blocking.