User blog comment:B214/Question and Answers/@comment-79.49.169.68-20121231205007/@comment-33447191-20130101005435

For Sacred Gears with entities sealed inside, there is an independent seal that throttles the power supplied to the user. For Boosted Gear and Divine Dividing, this seal can be temporarily disabled to obtain the full power of the Heavenly Dragons.

However, since it is activated through the lingering negative emotions in the Sacred Gear left over from past owners, Juggernaut Drive rapidly drains the lifespan of the user. It is a berserk state where the user completely loses control and rationality. Juggernaut Drive is difficult to deactivate and most users die, destroying everything around them until their lives are consumed. - (From Animesuki)

See http://highschooldxd.wikia.com/wiki/Juggernaut_Drive