Guide rings define the path that particles travel along in the accelerator.
They can be right-clicked to add a 45-degree turn on one side. Another guide ring needs to be in the particle's path as defined by the guide rings. Depending on the speed, the allowed distance between guide blocks increases.
Redstone Switches
When a redstone signal is applied to a non-straight guide block, it becomes a switch block. When powered on, it leads particles straight; when powered off, it guides them into the original curved direction.
For detailed information on particle acceleration mechanics, see the particle accelerator guide.