Since at least 1938, Jefferis had been toying with the idea of a demolition or anti-tank weapon that would be ideal for irregular warfare. It was to work by having an explosive charge deform so that it had a substantial area of intimate contact with the surface of the target.[3] Then, when detonated, the effect of the explosion would be focused on a small area and would rupture an armoured plate much thicker than would otherwise be the case.[3] Sappers call such a device a "poultice" or "squash head" charge.[3] Jefferis enlisted Bauer and Schulman of the Colloid Science Department of Cambridge University, they experimented with lengths of bicycle inner tube filled with plasticine to represent the explosive.