Photograph showing sectioned British No. 106 direct-action percussion fuze, World War I vintage.
The introduction of this fuze, the first British "instant" impact fuze, early in 1917 provided the British army with a way to use high-explosive shells to blast away barbed wire without digging large holes - previous fuzes penetrated the ground before detonating. Fuze 106 would detonate the shell's explosive charge instantly on striking even strands of barbed wire, and hence all the explosive energy was released above ground. This also enabled the use of high-explosive shells against enemy troops above ground. This effectively made shrapnel shells obsolete in the British army.
This is an early model. Later models incorporated a safety shutter.
The safety cap over the head was unscrewed and removed before firing.
Action :

• The hammer projecting from the head of the fuze is held apart from the fuze body by 2 steel split collars around the spindle, and hence the needle is held apart from the detonator within the fuze. A length of brass tape is wrapped around the collars, holding them in place. A small brass weight is on the outer end of this tape.
• On firing, the spin of the shell causes the centrifigal force of the weight on the end of the tape to fling it outwards, the spin unwinds the tape and frees the split collars.
• Centrifugal forces causes the split collars to spin out, leaving nothing between the hammer head and the fuze head. The fuze is now armed.
• On striking the ground or encountering even slight resistance such a strand of barbed wire. the hammer is forced down, the needle is driven onto the detonator inside the fuze, igniting the powder magazine and then the shell's explosive charge, without delay.