Mag energy is created by draconium, generated almost like static makes electricity. Dragons have large amounts draconium in their bones, and thus are able to generate large amounts mag energy, enough to channel and manipulate into streams and projectiles. If the energy is pure enough, then the dragon's human can channel it too. There have been cases where humans have fired mag energy on their own, without a dragon, but they cannot do much damage and the mag energy produced is very limited. Mag energy is almost like electricity, especially in that if too much is loaded into your body, it can burn through your nervous system. But, it can be controlled fairly easily on the dragon's part, and can latch onto things to pick them up, throw them or turned into projectiles or even shields.