>>891387They're really thin blades and have sharp hardened teeth. Worth mentioning is that because they are hardened/heat treated, you usually can't sharpen them, and need to replace blades when they wear out instead of being able to file and reuse them.
As for why pull and push aren't exactly the same thing but reversed, imagine you have like a wet noodle or something, and you're holding one end of it while the rest of it rests on a table. As you are pulling it along a table, it will straighten itself out vs if you tried pushing it forward it would bunch up. Kind of a ridiculous example, but i hope that illustrates the idea that with pulling, you are putting tension on the flexible blade. Pushing requires the blade to have more rigidity on its own, so it requires a thicker saw, which requires more work, but which also does a better job of staying straight without good technique because the blade is so rigid by itself.
If you took a thin pull saw, somehow attached a handle to the wrong side of it, and tried to use it as a push saw, it would buckle, because it's thin and floppy. Backsaws can make for a stiffer thin blade, and there are examples of both push and pull saws like that, but the rigid back support thing can get in the way of some cuts.
