Actually, these were not grenades (as in mortars or guns), but mines.
IIRC, there were 2 - 4 containers in the aft cone of a He 111. These containers would open upon release and disperse a 200g mine hanging on a cable and a small parachute. A following plane should pick up the cable and the drag of the 'chute would move the mine towards the plane. The mine would detonate on contact.
Some combat tests were run, but were not that successful.
Thank you ChrisMAg2, that makes more sense. I remeber reading about that configuration a while back.