An elderly relative was in the Home Guard in Liverpool during the war (just don't mention Dad's Army).
I remember him telling me that aerial mines descended on a parachute and the trigger/detonator switch 'unwound' on a length of wire and hung 100 ft. or so below the bomb on its descent. This resulted in the bomb exploding 100 ft. in the air (obviously) and causing much more damage - "they would take the whole bloody street out". How accurate this is I don't know.
As for the butterfly bomb, he said they were only used for couple of weeks that he knew of. They used to cause little damage but vast disruption all over the place whilst areas were made safe -- a combination of timers and anti-tamper devices made them nightmarish to deal with, and if the Germans had realised how effective they were they would have used them much more.