The threads on the camera body and the end of the nosepiece (short tube that fits in your 1.25” eyepiece drawtube or visual back) both accept the reducer. In theory, it could be attached three different ways to the camera. The recommended way to use the reducer is to replace the nosepiece with it. Unscrew the nosepiece and screw in the reducer.
On a related issue, you can use either the NexImage or the combined camera-reducer either with a star diagonal or without one. Some scopes may have back travel or back focus issues in one configuration or the other. Both ways will easily work with Celestron Schmidt-Cassegrain optical tubes.