take the tray apart and you will see that it is only a fancy box with 6 resistors mounted inside
the light blubs are the cut off device!!!
they go out at 0.9V i guess
that's why they suggest the user to remove NiMH packs when the lights go out, but not sure how the euqalizing part is done if the lights do not all go out at the same time since all the tray is still discharging all the cells as long as the cells are on the tray connected