Boost Gauge

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

In the Sonic Rush series
In Sonic Rush and it's sequel, Sonic Rush Adventure, the Boost gauge served the same function; You you fill it by attacking enemies and performing a Trick. Once it's filled a certain amount, you can press the Y button to Boost. It can be used to progress through certain parts of the levels, either by destroying weak walls, giving you enough momentum to use certain stage gimmicks, and is the means in which you enter the Special Stage as Sonic, by boosting whilst holding onto a strange machine until you are transported to the Special Stage. If you filled the gauge entirely, you would have the ability to Boost without depleting the gauge.

In Sonic Unleashed
In Sonic Unleashed, the gauge filled in much the same way as before. However, this time you do not get an infinite boost for filling it. You can however earn more sections on the boost gauge by collecting Rings in sets of 20. The tricks and points you get for defeating enemies in this game is now referred to as Action Links.

In Sonic Colors
In Sonic Colors the boost gauge worked largely the same as in 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 Unleashed.

In Sonic Generations
In Sonic Generations it worked the same as the in 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.