The Feng Shui Engine was implanted in Juri's left eye socket when she joined S.I.N. It acts as a replacement for her left eye, which was badly damaged when her family was attacked and her parents were assassinated by Shadaloo. The device was developed by S.I.N. and it enhances her fighting potential by emitting a very powerful ki.
The limits and the full potential of the engine have not been fully revealed, however, as shown in her first and second Ultra Combos in the Street Fighter IV series, it grants her amazing speed, strength, and very quick reaction times.
In the 35-minute Super Street Fighter IV OVA (Original Video Animation) which focuses on Juri's past, she quickly overcomes Chun-Li, Guile and Cammy at once while being under its influence. The anime also shows that it cannot be used for extended periods of time (likely reflected in the time limit of her first Ultra Combo in the games); this limitation however was later removed in the animation by S.I.N. scientists, replacing her Feng Shui Engine with an improved one.
In Street Fighter V, M. Bison defeats Juri at the Shadaloo base and takes her Feng Shui Engine. A few months later she finds a replacement eye in an old S.I.N. laboratory. The device's name (Feng Shui Engine Type Alpha) and the black and white tint of her moves suggest that the replacement eye isn't as advanced as the original Feng Shui Engine.