According to various manuals, and depending on the fuze (it's not a 'fuse') model, it can be a 'cotter pin' (a split pin in UK terminology), or a 'safety wire'.
In both cases this pin/wire blocks the movement of a spring-loaded setback pin. This setback pin in turn stops the movement of a spring-loaded safety pin, and this in turn blocks the spring-loaded detonator slider's movement.
Operation
Before use the pin/wire is removed and the mortar bomb (shell in US terminology at the time) is dropped down the barrel of the mortar.
On impact with the firing pin assembly in the base of the tube (well in the tube's base sealing cap) the percussion primer in the mortar bomb's ignition cartridge is initiated, its output igniting the smokeless propellent held within it.
The high pressure and temperature gaseous output of the ignition cartridge eventually breaks its cardboard walls. The gases then exit via the vents in the tail boom, igniting any augmenting charges present between the tail fins.
The gases from the ignition cartridge and or additional gases from the augmenting charges pressurise the area around the tail area of the bomb until it overcomes gravity (the bombs weight) and friction of the bomb with the tube's sidewalls.
At this point the bomb is rapidly accelerated up the tube's bore. The high setback force (acceleration) the fuze experiences causes the spring-loaded setback pin to move back against its freeing the spring-loaded safety pin.
The now free spring-loaded safety pin moves outwards (perpendicular to the axis of the bomb/bore) until it rests against the inner surface of the bore. This stops the spring-loaded detonator slider's movement whilst in the bore.
On exit from the bore, the spring-loaded safety pin is freed and pushed by its spring fully out of the fuze.
This frees the spring-loaded detonator slider, which is pushed across to its arming position by its spring. This aligns its stab-sensitive detonator below the striker assembly and its firing pin, and above a tetryl high explosive lead (transfer) charge.
At this point a slider lock pin is pushed, by its own spring, up and into a hole/notch the detonator slider, locking the slider in place.
The fuze is now fully armed.
Images
Film adaptation
To circumvent the fuze's safety features, as per Saving Private Ryan, the cotter (split) pin or safety wire is first removed.
The base of the bomb (shell) tail fins are then struck against a hard surface, making sure the percussion primer isn't struck.
If the acceleration is high enough, the spring-loaded setback pin moves back against its spring and frees the spring-loaded safety pin.
As the bomb isn't within the mortar's barrel, the spring-loaded safety pin would ping off to the side, freeing the spring-loaded detonator slider.
This rapidly moves into its armed position and is locked into place by slider lock pin.
At this point the fuze is armed.
Now armed, a strong enough impact with the striker cap, and hence striker assembly below it, will compress its spring and drive the striker's firing pin into the stab-sensitive detonator in the slider.
This initiates the detonator, its output initiates the lead, the lead initiates the booster charge, and the booster charge initiates the bomb's main high explosive filling.
Obviously you do not want to bang the striker cap on anything once armed and you'd need to throw the bomb so its trajectory is correct and that its impact speed is high enough to initiate the fuze.
