That's a maneouvre known as
Pugachev's Cobra. It's increasing the angle of attack (the difference between the orientation of the plane and the direction of travel) sharply for a brief period in order to bleed speed. In fact, every plane can fly at angles which are not the direction of flight, just nowhere near as extremely as this. It's how landings are made, often - the plane is moving in a straight line, but the nose might be pointing ten or fifteen degrees off to the right or left.
Even more impressive is the ability of one of the later model Su-27 variants to perform a 360 degree flip inside its own length in level flight. I wish I could find video...
Disclaimer: the bits and pieces I know about flying do not come from much real piloting experience, so the details may not be entirely accurate. Do not attempt to use this information to engage in aerial combat, or land a plane.