The instructions told soldiers to use a lit cigarette for the job. The fuse smokes until it burns to the level of the heating element.
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From the link you provided:
According to those who returned home after the War, soldiers fighting in colder climates often tucked the hot cans inside their jackets as substitutes for hot-water bottles. The can itself was too hot to touch without a cloth around it.
When it first released the product in March 1944, the British Ministry of Food did not give out details of the precise nature of the heating element for security reasons.
The BBC’s WW2 People’s War Archive contains a story from Lawrence McHugh about the new invention. He told a volunteer that:
"We were waiting off the coast of Southern England on June the 5th 1944 and we were quite tense tired and worried. The officers said we’d get some food out and we’d try this – something we haven’t seen before – self heating soup. The only flavour, I think, was Tomato. In the top of the metal tin was a little circle and it said ignite using a cigarette – but before this there were two little arrows on the side of the can where it said punch here and here. We all punched a hole in both sides, apart from one man who punched the holes in the side of the can. The cans were really hot as the fuse went all the way down to the bottom. This man had put his soup to one side on a ledge which was level with his head.We all had cans with the soup coming up from the top but because he’d punched holes in the side of his can the soup came out at force of the sides of the can – straight into his ears!! He had tomato soup all down his ears but unfortunately for him he became our first casualty with a badly scalded ear.
The whole ship was quickly laughing and the story spread down the ship like wildfire. Other ships which were part of the flotilla quickly heard the laughter and were asking what we laughing at and the story quickly spread down the whole flotilla. It was a tension relieving moment in the waiting."
Another soldier, Raymond Eaglen, remembers being posted in Normandy where the cans often exploded, showering hot soup on “anyone within range”.
But they were often very welcome comforts. Ted Dann recalls one night outside Bremen.
"There were a few incidents encountered after crossing the Rhine, but the war was rapidly coming to the end, I recall we were engaged in combat outside Bremen, there was a lull in the activity, we were all feeling a bit tired and quite hungry, five of us in this tank waiting to go on or pull back, longing for something to eat, I suddenly remembered the five tins of self heating cans of soup we carried on board. I mentioned to the corporal in charge we had these soups, he said they were only for emergency use, I reminded him we had not eaten for hours and surely this is an emergency!“There was silence for a few minutes, then he relented and said “get them out then”.
The soups were in the standard size tin, with an I.C.I. element in the centre, the idea was you pierced the tin, pulled on the central flap which ignited the heating unit, and within a minute or so the soup was piping hot. They were delicious.
We made a mental note to get them replaced sometime, but we never did, in fact we never saw these items again,they may have been withdrawn, they could have been a danger if you forgot to pierce the tin, going up like a small bomb no doubt, perhaps they were just on trial, anyone know of these? They were certainly most welcome that night."