sessions and garbage collection

I think I'm not understanding the garbage collection concept with sessions. When a session is created, a session file is stored in my /tmp directory. What I want to know is, does this gc process delete these files once they are older than the session.gc_maxlifetime? What do I have to do to invoke the gc process? Is this a manual process that I must take to delete these session files? I've set the session.gc_maxlifetime to 10 seconds just to see what might happen.