Sebastian Shaw's power is the ability to absorb energy in pretty much any form, such as removing the kinetic energy from a bullet as it comes in contact with his skin (no more kinetic energy in the bullet means his skin is enough to stop it as there is no more force behind the object to continue to add velocity), or the radiation (pure form of energy) from a reactor.
Also note that apparently there is a limit to how much energy Shaw can absorb at any given instant in time (a resistance, if you will, possibly measurable). Examples of this include Shaw's repeated acts to absorb power from the sub's reactor, and the time taken to absorb the entire grenade blast.
Magneto possesses the ability to control metal objects (such as a coin) as though he were Telekinetic (with the restriction of only being able to affect specific metals). In effect, Erik uses his power to apply just enough force to the coin to slightly overcome the surface resistance of Shaw's skin/forehead and to overcome the instantaneous capacity of Shaw's energy absorption, and then keeps applying that force, even as Shaw is draining that same energy away. If there was a way to visualize what was happening, there probably was a massive amount of total energy exerted by Magneto and absorbed by Shaw, but by carefully keeping the force behind the the coin's movement at a level he could continually produce faster than Shaw could absorb it, Erik was able to keep the coin moving into Shaw until he'd actually killed him, proving that while Shaw could absorb huge amounts of energy, he wasn't really able to do all that great at absorbing 5.67 grams of Cupro-Nickel composite.