At left is the fuze ready to use. The curved wire safety pin locks the set-back pin "A" in position. That, in turn, retains the bore safety "B" which keeps the primer (red) off center, clear of the firing pin .

The safety wire is removed just prior to use. This has no direct effect on the fuze.
When the round fires, inertia drives the set-back pin "A" down, overcoming its small creep spring, releasing the spring-loaded bore safety "B". While inside the mortar tube, this safety is still held in position by the tube wall.
Once the round clears the muzzle, the bore safety and spring departs. "C" is now free and is pushed into position, centering the primer under the firing pin and aligning with the detonator "E". The round is now armed.
On impact, the striker "D" hits the primer, which sets off the detonator. The booster element in the cup follows, which detonates the main HE filler in the round.

In the Hollywood movie, "Saving Private Ryan", there is a battle scene where 60mm mortar rounds are used as improvised hand grenades, manually armed by striking the bottom of the round, hard, against a steel plate. (Function position "A" above.)
While this technique was indeed used during the war, manually tossing the round overhand is likely to cause it to tumble end over end. If the striker does not hit the target, it will not go off. Dropping the shell from a rooftop, or window would be a practial method of use, in a "last ditch" application.

Newly Painted and Re-Stenciled M49A2 Rounds