Steel is an Allomantic metal and an alloy of Iron. When burnt, Steel allows the user to telekinetically push against a metal object, presumably propelling that object in the opposite direction. Normal laws of physics (Force = Mass * Acceleration, and the conservation of momentum) still apply here, so if you push against a massive object, you will need to apply more force to make it move. If you push against an object similar in mass to you, you may move as well, and if an object is much more massive than you, you will be pushed away from it. As a result, many allomancers use steel to allow themselves to fly through the air. A common use is to drop a coin on the ground, and then push on it. Because the coin is against the ground, steel-pushing on it is effectively the same as pushing on the ground, which is exponentially more massive than the Allomancer, so the resulting acceleration primarily affects the allomancer, allowing them to "fly" through the air. Allomancers also push on coins to use them as projectile weapons. Other uses for steel-pushes include pushing upon a person that possesses a metal object, or disarming a person with a metal weapon. Any misting only capable of burning steel is known as a coinshot.
When used in Feruchemy, steel allows the user to store physical speed. In exchange for being slower now, they will be faster in the future.