It seems like a simple enough question that someone’s more than likely to already have asked, but why does smacking a knife or blunt utensil end on the top of a jar help to loosen and remove the lid? My Gran does it all the time but frankly I don’t quite get it. Generally, when I hit downward on something it helps to keep it in place or pushes it down further. I’d really enjoy getting someone with better reasoning than I to answer this; I’ve been batting this one around in my brain since I was a kid and still haven’t got a clue.
The shock loosens the seal a bit … enough to twist the lid off …
Hot water is another way … the metal lid expands more than the glass which makes it easier to twist off …
Jar lids stick for a couple of reasons. The most common I find are because there is a vacuum inside, because of friction, or because something has dried and glued the lid to the jar (think jelly jars or honey).
Smacking the top of the lid can break a vacuum by deforming the lid enough to let a bit of air in. Or it can loosen any glue. Rapping the edge of the lid with a table knife can do the same. Running under hot water can cause the metal to expand, or it can also dissolve the material sticking the lid to the jar.
Tapping on the lid addresses several possible causes for the lid being stuck including those mentioned above and slightly expanding the metal enough to relieve tension just from being screwed on too tight.
My brother used to tap on the flip top of Coke cans before opening them. Said it relieved the pressure and averted sprays.
I always thought this was just a useless habit, superstition, or affectation (I got a million more where those came from :)…).
Was he, God forbid, right?
ETA: just realized this is a hijack. Not worth an OP…
Leo - tapping the lid doesn’t do jack. What it does is allow some time to pass so the carbon dioxide can return into suspension.
Oh he was right
He was just in reverse. 
Tapping it just ads MORE energy, adding to the foam over.
Only way to reliably fix the problem is wait.
Ive seen people lucky enough ti just barely crack the seal to bleed off pressure when they are pressing on the tab but close back up when they let go, and bleed off the foam over that way, kind of like loosening a 2 liter cap till the bubbles rise, then closing it till they sink, and repeating a few times.
I do not have that luck, so i wait for the bubbles to reabsorb.
It breaks the vacuum seal.
Back to the OP: If the jar lid is one that twists off with just a short turn here’s another trick. Like olive or pickle jars, but not so much like mayo or peanut butter jars that unscrew.
Take an old-fashioned “church key” bottle opener like this: https://www.nepartysupplies.com/wp-content/uploads/2013/12/4-church-key-can-and-bottle-opener.jpg. Using the non-pointy end, hook the little tab under the skirt of the lid then gently lift the skirt of the lid just a smidgen. You’re trying to flex the edge of the lid, not bend it. You’ll hear a hiss as the vacuum releases. After that the lid twists off much more easily than it would have with the vacuum still intact.
Banging the lid is just an indirect way to do much the same thing.
Here’s an action pic of the same idea with a different flavor of opener. Just imagine a jar lid in place of the bottle cap: https://qph.ec.quoracdn.net/main-qimg-aa4d71ae5b5cde8fdb7b0a2ec32d86b0-c
The advantage of a church key for this is they can generally fit around lids with taller lips than the kind in the second pic. The more modern ring-style bottle openers like https://www.webstaurantstore.com/images/products/main/27209/423271/flat-bottle-opener.jpg are completely useless for this trick; they can’t engage a jar lid at all.
