mrjenrst the impact of dropping the mortar in the tube is enough to detonate the firing charge. its not rocket science. erm ok it is

You will find with most munitions a small charge sets off the bigger main charge, be it a warhead or a propellant. All it takes is a small amount of impact sensitive explosive and a impact to set it off.

Its really not that complicated.

If you can understand that a toy cap gun, the impact of the hammer sets off the bang then you can understand how most munitions and explosives work.