To use it, simply hold down either the duck key (S or ↓).
The longer you duck, the more force you accumulate, as is seen in game as more smoke appears at your feet, and your vibration intensifies. Higher levels of this skill affects how high you can rocket jump and how fast you can charge it.
Moving or jumping will cancel the effect, the boost is used when you stop ducking.
The jump pushes you a bit to the right or left.