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In that example, the bottom of the black part is designed to fit nicely in a 2" focuser. It's just a convenience thing for some people who have big 2" eyepieces and don't want to fiddle with an adapter when they switch from 2" to 1.25" eyepieces. It doesn't change the performance of the eyepiece.
Note that it works fine with a Newt/Dob but if you have a scope with a star diagonal you need to make sure the chrome part of the eyepiece has enough clearance so it doesn't hit the mirror in the diagonal.
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