The Imperial shape is alson known as the traditional or classic yo-yo shape. Out of the three shapes the imperial shape is primarily used for looping tricks.
The butterfly is assentially the imperial shape reversed. It looks a bit like the separated halves of a standard yo-yo that have been reconnected back-to-back. Butterfly yo-yos have a wider string gap that makes it easier to catch the yo-yo body on the string. The butterfly shape is best used for string tricks and is not commonly used for looping tricks because the winged shape of the body does not allow it to easily flip while looping.
The modified is also known as the Modern and Proyo shape. It is a great combination of both the the butterfly and the imperial yo-yo shapes. The rounded rims make for smooth, easy landings for string tricks. It also has a slim profile making looping easy as well. This yo-yo also has additional rim weight which allows the yo-yo to sleep for long periods of time.
Looping is a yo-yo technique which emphasizes keeping the body of the yo-yo in constant motion, without sleeping. Yo-yos optimized for looping have weight concentrated in their centers so they may easily rotate about the string's axis without their mass contributing to a resistance due to a gyroscopic effect.
Keeping a yo-yo spinning while remaining at the end of its uncoiled string is known as sleeping. Sleeping is the basis for nearly all yo-yo tricks other than looping. Most modern yo-yos have a transaxle or ball bearing to allow the yo-yo to sleep longer. As of 2010, the world record sleep times were 3m51.540s for fixed-axle and 21m15.170s for transaxle yo-yos.
In off-string play, the yo-yo's string is not tied directly to the yo-yo's axle, and the yo-yo is usually launched into the air by performing a Forward Pass to be caught again on the string. Yo-yos optimized for off-string tricks have flared designs similar to the butterfly shape. This makes it easier to land on the yo-yo on the string. Yo-yos for off-string play often have soft rubber rings on the edges.
In freehand play, the yo-yo's string is not tied to the player's hand, instead it is tied to a counterweight. The counterweight is usually a dice but it can also be anything from a beanie baby to a cell phone. When playing the counterweight is thrown from hand to hand and used as an additional element in the trick. Freehand is now the fastest growing style of yo-yo play.