The 10 PSI means, that approx 10 PSI is the minimum of boost pressure your engine will see. If you hook up a hose onto the wastegate fitting, apply 10 psi, the wastegate will start to open.
So anything over 10 psi of boost pressure, the wastegate will be open, & will be vented to wherever you have plumbed the wastegate piping to., Back into the downpipe, separate mini wastegate exhaust system.
Note: If you plan on having the wastegate exhaust plumb back into the downpipe, you will want to have it join into the downpipe further back( further away from the turbine wheel) Minimum 18" downstream of the turbine wheel.
The way the exhaust enters the wastegate is important also as important the way the wastegate exhaust is introduced into the downpipe.
Minimum boost pressure can be 5 ,10, or 15 PSI, by changing out the wastegate springs accordingly.
I would get one (wastegate) that has a few wastegate springs so you have the ability to adjust whatever boost pressure you would like. 10 PSI of boost is a bit high, just to start off with. You need to start off with a lower boost pressure, just to be safe & gradually add boost pressure when you get your tune figured out better.
