Its braking, while braking you use you right foot in a way to blip the throtle to match revs while you downshit.
Basically its all 3 pedals at once, use your right toes on the brake and then push you right heal forward to blip the throtle for the rev matching (i can do it by rolling my ankle in my car to have just enough shoe to jab the gas), while all of this is happening you are downshifting with the left foot. When completed properly its a beautiful thing. Its also hard to explain and takes a little time to perfect. I will see if i can find a link.
Here is a link i only looked at it for a second but i think its allright.
How to heal and toe downshift