Boost Gauge

The Boost Gauge is a function used in the Sonic the Hedgehog series to inhibit the use of Sonic Boost, and in some games perform a special action when filled.

Sonic Rush/Sonic Rush Adventure
In Sonic Rush and its sequel, Sonic Rush Adventure, the Boost gauge served the same function: the player fills it by attacking enemies and performing a Trick. Once it's filled a certain amount, the player can press the button to Boost. It can be used to progress through certain parts of the levels, such as destroying weak walls, giving the player enough momentum to use certain stage gimmicks, and is the means in which you enter the Special Stage as Sonic via boosting whilst holding onto a strange machine until Sonic is transported to the Special Stage. If filled the gauge entirely, the player would have the ability to Boost without depleting the gauge. Taking any damage depletes the boost.

Sonic Unleashed
In Sonic Unleashed, the gauge filled in much the same way as before. However, this time the player does not get an infinite boost for filling it. You can however earn more sections on the boost gauge by collecting Rings in sets of thirty. Taking any damage still depletes the boost. The tricks and points you get for defeating enemies in this game is now referred to as Action Links.

Sonic Colors
In Sonic Colors the boost gauge worked largely the same as in Sonic Unleashed, however, it's explained in game that the boost gauge is filled by White Wisps. You can collect capsules of trapped white wisp to increase the boost gauge, however, the gauge does not increase like it did in Sonic Unleashed.

Sonic Generations
In Sonic Generations it works the same as the in Sonic Colors, but with the lack of wisps to fill the gauge, it reverted back to the action link explanation, and there were no capsules containing more boost energy.