I've been trying to understand the Blandford–Znajek process which explains how Gamma Ray Bursts form, but it's still not clear to me how it works. In this article, several theories about the formation of GRBs are mentioned, and in the 4th page, the author talks about the Blandford–Znajek process.
In that part he mentions: "These rotating field lines induce an electromagnetic force that accelerates charged plasma at relativistic speeds along the axis of rotation. Due to the radial component of the field, the particle spirals as it leaves.", and there is a figure taken from Kip Thorne's book Black Holes and Time Warps illustrating this. I'm having trouble understanding this figure and why the particles spiral as they leave. Does anyone know what he's talking about? If you know any equations involved in this I would appreciate it.