Have a look at this video for a basic understand of a simple "pulse counting" detector:
https://www.youtube.com/watch?v=jQlN2fc7LJc
The delay line in the Fisher is not terminated, watch this video to see what happens when a transmission line is not terminated with anything:
https://www.youtube.com/watch?v=I9m2w4DgeVk
Then bear in mind that whatever is flowing in the delay line is coupled by the delay lines capacitance to the next stage.
Lawrence.