It is a spring-loaded firing pin. It replaces the whole firing pin, and has a spring. Early model (49 and some 50) SKS had a spring loaded firing pin, but they decided to eliminate it for whatever reason.
The Murray has a mediocre reputation though. I have heard of numerous reports of pins breaking. When I first got my SKS last year, I did tonnes of research as to "upgrades" for it... And didn't end up doing a damn thing to it. Removed cleaning kit/rod, and bayonet, and I bought a slip-on recoil pad from Walmart for like $11 to increase the length of pull. Thats it. I want to get a red dot or reflex sight on a scout mount one of these days, but other than that Im leaving it alone. An alternative to the red dot would be a tech sight and a skinny front post... But I have been hesitant about doing anything that is a permanent modification to the rifle...
Apparently there were some of the Murray's Generation 1 firing pins with spring had problems but apparently the Gen 2 has the bugs worked out.
A properly cleaned SKS rifle and bolt should not slam fire but if the bolt gets too gummed up that the firing pin starts to stick in the forward position then it will slam fire. The pin springs should prevent this but are not an excuse to not properly clean your SKS after each use especially if you are using corrosive ammo.
Murray's firing pins can be ordered directly or from HI-Cal one of our site sponsors. Lever Arms has a few available already installed in some SKS' offered. Not sure if Weimajack or others offer this option.
Suther makes some good points buy an SKS keep it thoroughly cleaned and oiled and do not waste any money on any upgrades because it will always be a $200.00 SKS.