This is a twin panel frame with worker foundation in the
top part and a void left in the bottom portion.
The idea is that mainly drone comb will be drawn in the
vacant space at the bottom and then this can be cut out and destroyed
as a means of drone sacrifice and varroa mite removal.
I tried some out in the Summer of 1994... The bees drew out
the worker foundation as worker comb and filled the blank portion
with mainly drone cells. So it worked OK as far as this, but I gauged
that the number of drones (and thus entrapped mites) that were culled
was not sufficient.
Many UK beekeepers just place a shallow frame in the side of
the brood nest and then trim off the wild comb that is built on the
bottom edge of this frame. This may remove a few more drones, but I
am still sceptical that the numbers are insufficient to fullfil the
objective of removing enough mites.

The components making up this device are:-
a 28
mm wide topbar,
two x 38 mm wide Hoffman side bars having the
reduced section 28 mm broad. Each with two 8 mm wide x 6 mm deep
notches 139 mm from the top edge,
four x Manley style bottom
bars,
A piece of shallow worker foundation is fitted in the
top window of the frame.
If the notches
are 134 mm from the top edge instead of 139 mm, then the bottom
edge of the foundation protrudes slightly through the mid bars and
provides a guide for the drawing of the drone comb.
Originated... 20 Feb 2002