Your description is of magnet only reversing momentum you yourself have created. Is it not possible to have it work off other momentum?Ah, I think I know what you mean. As mentioned above, knockback and movement momentum are tracked independently. You also fall during knockback and magnet stops that fall, making you shoot upwards when using it while still suffering from upwards knockback. If you'd already begun drifting towards the stage and b-reverse magnet, you'll also reverse the horizontal movement you had towards the stage, but not the knockback component, and thus also gain a boost horizontally away from the stage, because you're now both moving and being knocked away from the stage.
I think the point I was getting as was that I was still moving horizontally away from the stage (while DIing towards it of course) but I get to a point when I am no longer unable to input a command to my character (which I don't know a good term for). It is at this point that I hit down-b and slide the joystick 90 degrees back toward the stage. Since my momentum is still moving slightly away from stage (it has not gone past neutral and toward the stage yet), my input is considered a wavebounce[?] (e.g., I'm floating to the right -> down-b -> left within the first couple frames) and pushes me farther away from the stage (a la: a small magnet pull?). Again, I haven't had time to test this since when I posted it originally, so I could still be making things up...but I think that's what's happened.