A Shimmy is a tactic in the Street Fighter series.
A shimmy is a tactic that involves going in and out of a range of an attack. All attacks in the Street Fighter series have recovery frames that can be avoided with this tactic.
Shimmies have existed throughout the existence of Street Fighter. The effectiveness varies from game to game. Usually the power of shimmies depend on game to game and even character to character. Most of the time it is done against Grabs due to their recovery.
Usually characters that benefit the most from shimmies are those that have high backwards walk speed or long range pokes. Shimmies are very effective against short range normals such as Ryu's Crouching LK. Shimmies are most effective against Grabs due to their usually short range. By walking forward a bit you can trick the opponent to try and tech a throw. Usage against grapplers is dangerous as their command grabs tend to have the range to stop this tactic.
This tactic has reached the pinnacle of its usefulness in the Street Fighter V series. In previous games either throws were linked to a fierce (they usually have a good amount of range) or in games like Street Fighter IV most players preferred to use "Crouch Techs" an Option Select where instead of doing the throw whiff animation, the characters would tech a throw if the opponent attempted to throw and if the opponent did not throw, the character would instead do their Crouching LK. as shimmies depended on standing backwalk it would leave the user susceptible to be hit.
Crouch Tech was removed in Street Fighter V forcing a throw tech attempt to instead do the full grab whiff animation. It is recommended for the user to learn the grab ranges of their opponent's character as well as the grab hurtbox of their own character to maximize usage of shimmies.