That is Critical Capture.
One final new battle mechanic introduced in Black & White is the critical capture. When a PokéBall is thrown, it has small chance to become critical, which significantly increases the capture rate, about 20 times the original. The PokéBall will flash and slow down in the air before landing. Then it will shake once before confirming the capture. It doesn't guarantee capture in all cases but it rarely fails.
BTW, master ball also shakes 3 times before capturing.