PSX-AR block detection question

[Mr Bones]

I have a PSX-AR which controls the return loop (very well I might add) and also a DR4088RB-CS outside of the return loop for block detection in the fiddle yard. The manual says that the PSX-AR can also provide block detection via current draw, but I can't for the life of me figure out how? All this is controlled by a Z21.

Has anyone done this? Any advice welcome