you drill holes in the filler neck to let the boost escape more easily because it will have more than one way out (the over flow nipple)

 

wouldnt that just create a boost leak though instead of just making it not get to a certain psi, or does it not come out until you reach that certain psi (13 in this case)

 

The spring in the rad cap is set to 13psi. So you can boost upto 13psi and it won't ever open. You won't even know its in there. But just incase something happens this will save your *** and bleed off the rest of the boost once it hits 13psi. In my case it helps control my initial boost spike I get with my T25.