The nick you're describing creates what structural engineers call a 'stress riser' in the material of the bag. It focuses the force you exert on the bag to a small point, from which the tear will originate.
Stress risers are a common cause of structural failure, and are usually designed around. Some well known examples where they cause failures: rectangular window openings in aluminum fuselages, knots in fishing line, a poorly designed head tube lug on a steel bicycle frame.
In this case, however, the failure of the material results in a good outcome: snack time.
I see them over here all the time... you'll find them on jerky packets, for example, or as the initial seal on nearly any re-sealable bag (think of a bag of cereal with a zipper on top). The problem is that if the Seal God (blessed be his flippers) doesn't smile on you, you'll end up ripping off a sliver of the top of the bag and completely failing to get a decent opening. This has caused much wailing and gnashing of teeth. I think the material used for the bag/packaging is definitely a factor--I can pretty much always make a good tear on a ramen flavor packet, but I don't know that I've ever had a bag of jerky open properly.
God, all this talk of cereal and jerky and ramen makes it sound like I eat only crap. I eat vegetables, I swear!
They frustratingly don't often work with greasy fingers. Which you might have if said sachet or packet comes with, say, a fast food meal, or from inside a bag of crisps (like Salt n Shake in the UK).
Stress risers are a common cause of structural failure, and are usually designed around. Some well known examples where they cause failures: rectangular window openings in aluminum fuselages, knots in fishing line, a poorly designed head tube lug on a steel bicycle frame.
In this case, however, the failure of the material results in a good outcome: snack time.