When opening a glass jar of prepared spaghetti sauce, (with the seal flat), and it makes a loud sound when opened, is that bad or good?
When being canned (or jarred as in this case) the ingredients are put into the jar hot before a lid is put on. When the ingredients cool it creates a suction that pulls the lid down. When you open the lid and hear a pop what is happening is that air is coming in to fill the low pressure inside, which indicates that there was a good seal. This is definitely a good thing.
If there is no pop that means that there is no longer low pressure in the jar, this could be because it was never sealed properly, or the contents have gone off despite the seal and created gas inside the jar. Either way it means that it's not safe to eat.
The popping is used to indicate 1 of 2 different things have NOT occurred.
1) has it be opened previously or didn't have a good seal.
2) spoilage, even if the seal was perfect, if spoilage occurs it will create gasses that will increase the pressure inside the jar.
Caveat: we had one question in the past where someone opened a jar of sauce, used a small portion and within an hour or two fizz started pouring out of the jar from the unused portion, so spoilage can still occur without indicators. I think that was a 1:1,000,000 occurrence, but it can still happen.
The pop is vacuum sealed for freshness through heating, steaming, homogenizing, pasturizing. It pops when you break the seal like popping a can of coke or pepsi. No pop, no seal, not fresh (probably spoiled). There are pops for both vacuum seals and pressure seals.
Vacuum seals suck the air out. A pressure seal is hot food, sealed, then placed in cold bath.