The Pistolet Makarov has a free-floating firing pin. A free-floating firing pin does not have a spring. Because it does not have a spring, there is only one way to ensure that the firing pin will be in position for the hammer to hit it; the firing pin must be longer than the firing pin tunnel. When a cartridge is in the chamber, the cartidge prevents the firing pin from moving so far forward as to be out of reach of the hammer.
If you manually lower the hammer, the hammer will be left resting on the firing pin, which, in turn, will be resting on the primer. A blow to the hammer will fire the pistol.
The decocking safety intercepts the hammer fall and locks the hammer before it reaches the firing pin. When the safety is subsequently switched to "fire", the sear holds the hammer off of the firing pin by engageing the hammer's safety notch.
Use the decocking safety!
Because of the free-floating firing pin, it is essential that the firing pin tunnel be clean. A tunnel filled with decades-old congealed cosmoline can cause a slam fire if the firing pin is in the forward position with its movement restricted.
In a properly operating Makarov the firing pin can mark the primer as a round is chambered, but does not have enough mass to fire the round.