I agree. There is a lot of energy even in small firecrackers. When they are set off in the open air that energy works on the atmosphere an can expand and disipate quickly. At a rate of approxamately a cube root over distance.
But if it is contained that engery is going to work on what is containing it, in this case his hand.
On the flipside, that’s what make firecrackers and soup cans so much fun.