Most snips have a single pivot, and the cutting of a metal sheet is pretty much a matter of brute force on the handles. The force applied by the cutting blades against the metal is a simple mechanical ratio between the length of the handles and the distance from the pivot point to where the blades make contact with the metal.
The aviation snips have a second pivot point that allows the handles to apply a "multiplied force" to the smaller "headpiece" snip asssembly, so that the force applied to the handle is multiplied, usually by about 4x at the point where it's applied to the "handle end" of the actual cutting blades, which have an additional - usually about 1.5x to 2x - multiplication of the first force to the actual cutting force at the blade. The end result is that with the aviation snips the same handle effort applies close to 8 times the effect at the cutting point on the blades as would be obtained with a pair of single-pivot snips.
The "compound lever" effect is similar (conceptually) to using an enormous pair of pliers to squeeze the handles of your simple snips together - and the same effect as using simple snips 8 times as long as what you've got.
When you shear something the material on one side of the blade has to go in one direction and the stuff on the other side has to go the other way. When cutting sheetmetal (or the &#$@! #@$@! that HP puts their ink cartridges in) the cut sheet "springs back" and may bind on the blades and also may make it difficult to shove the shears further along the cut to make the next squeeze.
A secondary feature of the aviation snips is that the blades are "exotically shaped" to create a small amoung of "crimp" to the sheet as it's cut, so that it takes less effort to retract the blades (re-spread the handles) and advance them for the next cut.
Especially with soft metals like aluminum or copper, the "springback" as the blades are reopened is enough to require some effort to get the shear off the metal after you've made a cut. In most cases the simple snips have "loops" on each handle so that you can pull them back apart after making the "snip," and with some materials it can require almost as much force on the loops to open the blades as it does to make a cut. The shaping of the av snip blades reduces the "withdrawal" force suffiently that a small spring is sufficient for the return stroke, and the loops are not needed.
Especially since it's not necessary to have the loops to reopen them for the next cut, the handles of av snips are perfectly symmetrical, so that it doesn't matter which-handed the user is to use either-handed snips. The compound lever construction also permits a very massive pivot at the actual cutting blades, so that they're stiff enough that there's no need to apply "hand torque" to keep them from spreading.
Aviation snips are commonly "rated" for about 18 ga (0.050") steel, but occasionally the packaging will say 16 ga (0.0624"). Because I'm old and feeble, for steel that thick I'd probably use a hacksaw (or a cutting torch if I had one), but I've recently cut a few short snips in some 1/8" thick fairly soft aluminum with little difficulty.
With simple snips of sizes I have*, cutting .06 thick steel probably would mean propping the snips up and hitting the handle loop with a 3 pound hammer. (And then I'd use a short crowbar to pry the blades back open.) It would be a little easier if the sheet was "dead annealed" but it's seldom found that way.
* I think I've got a 14" pair somewhere in a toolbox?
But when we get to the bottom line here, about the only thing we can conclude with reasonable certainty is that gnu didn't look at the "theory" link in my previous post.
He probably thought the link was to something too technical, but the picture's actually from an article about how to use tin snips (av style) to crack pecans..
Incidentally, I actually do keep an old pair of Dutchmans next to the spare ink cartridges, because I recently broke a pair of kitchen (bone cutter) shears trying to get a fresh HP Inkjet cartridge out of their ridiculous package.