you round your wrists out instead of in, and turn your hand over at the end. when you hit the back of your hand is facing yourself.
in this fassion the knuckles hit flush with the face with the larger knuckles landing on the tip of the chin (the button) while the rest of the knuckles catch the face.
http://www.youtube.com/watch?v=rHvPHmaqYF4
you can watch him hit the speed bag with his scientific rounded punches but their a bit wide and open, the last one he throws on the bag is in slow mo.
you can also watch me throw a few at about the 8:20 mark
http://www.youtube.com/watch?v=S4NI5xonqlQ