Beefcaking is a mod that involves adding an additional bearing to a yo-yo to widen its string gap to allow for advanced, low-responsive string tricks. Although most yo-yos can be beefcaked, very few can actually benefit from the addition of an extra bearing.
Beefcaking was invented by Doctor Popular in 1999 while working for the U.S. distributor of Henrys. At the time, yo-yo play was evolving into a less responsive style. Most yo-yos had adjustable gaps to allow for this new style, but the narrow bearing inside the Henrys Viper prevented the gap from being opened as wide as the other popular yo-yos at the time. By beefcaking the Viper, the yo-yo's gap could be opened twice as wide as before, allowing enough room for friction stickers to be added without making the yo-yo super grabby.
Beefcaking also has the extra advantage of centering the string (like a KonKave or Center Trac bearing) due to the groove created in the center of the gap, where the edges of the two bearings meet.
The name "Beefcake" was coined from an early episode of South Park, where Eric Cartman attempts to "bulk up" by taking Weight Gain 4000. In the episode, Cartman continues to refer to his new body weight as "beefcaked!".