Magnaflow mufflers employ a straight-through design (with no baffles) that is packed with stainless steel wool. A perforated pipe of the same diameter as the inlet and outlet runs straight through the muffler case unimpeded, and the stainless-steel wool surrounding the perforations soaks up the annoying frequencies like a sponge, turning the kinetic energy into heat, while passing the flow straight through the muffler.
This design does a really good job of absorbing acoustic energy across a wide band of frequencies — especially midrange and upper-midrange frequencies. This type of acoustic dampening takes the edge off the sound while preserving the lower frequencies — the "rumble." From a flow perspective, Magnaflow mufflers are hard to beat (a straight pipe or a bullet muffler would be a good flow analogue) and the Magnaflow sound is preferred by many.