Hi John,
I don't own one of these eyepieces but it looks to be a twist up eye guard similar to my 22 nagler.
These are adjustable to make it more a comfortable viewing experience especially for people that wear glasses so as not to touch glass on glass.
Using the twist up eye guard will reduce the field of view, optimal viewing for that eyepiece seems to be with it wound in.
I sometimes forget mine has the twist up guard altogether .
Enjoy
