An alpha shift is a "test shift" for a new BRC Ranger. Before you hit the playa, you go to training. Once you get there, you have to prove that you'll be a good ranger, so you undergo some stress tests and perform a dirt shift with a mentor Ranger. So if I pass my alpha shift, I'll be a Ranger this year!
(Black Rock Rangers are nonconfrontational community mediators. They go around helping out disoriented or distressed participants, resolve conflicts, and generally act as trustworthy authorities in cool hats. The goal is to intervene before law enforcement becomes necessary. Even though a Ranger's authority comes entirely through social capital and people listening to them, there's a shared incentive to allow Rangers to resolve issues before cops are necessary.)