BakuDouble Strike was a series of special Bakugan that were released exclusively during Bakugan: Gundalian Invaders. These Bakugan typically feature a BakuDouble Strike featured a button that causes a second transformation, revealing a second Double Strike Bonus G-Power.
When a button is pressed on a BakuDouble Strike Bakugan, they will undergo a second transformation, such as extending or opening a part of the Bakugan, or launching a small projectile. For example, when Snapzoid first opens, only its head and side pieces open. Then, after pressing the button on the back of its head, the thinner second head that resides inside of Snapzoid's neck comes out. A hidden G-Power will sometimes be revealed upon pressing the button. The exceptions to this feature are Dartaak and Sky & Gaia Dragonoid, where both of these Bakugan open automatically instead of requiring a manual input.
Dartaak follows this rule, now matter where the projectile lands.  However, Sky & Gaia Dragonoid act as two different Bakugan, where they counted as two seperate Bakugan if each piece lands on a separate Bakugan. 
BakuDouble Strike Bakugan
Avior- Extends neck and widens wings.
Phantom Dharak- Pushes wings down.
Blitz Dragonoid- Extends arms.
Dartaak- Launches "dart".
Phosphos- Activates four heads.
Sky & Gaia Dragonoid- Sky Dragonoid is launched out of Gaia Dragonoid and they are treated as two separate Bakugan.
Blitz Dragonoid's Button