Here is a diagram of Apple Jack’s basic lay-lines. this is how i understand earth pony magic to be shown. the main lay-line runs along the spinal cord and splits along each leg, culminating at a focal point at the base of each hoof. this allows earth ponies to deal massive blows with their hooves and control the elements of earth unlike the other sub-races except alicorns. like pegasi, this magic is always being casted, and does not eventually tire the earth pony. however the earth pony can focus more energy into a particular hoof, which increases the magical prowess of that hoof. this is why they can deal heavy blows with more than just bucking. however the action of bucking aligns the main lay-line and gives a single direct line to the rear hooves; allowing for a more focused magical attack.