Complex sentence: To begin descending the inrun, they first drop down from the gate to a crouching position, upon which speed is rapidly picked up within seconds via built-in tracks into which the skis are slotted.
Split into simpler sentences: To begin descending the inrun, they drop down from the gate to a crouching position. Speed is rapidly picked up within seconds via built-in tracks into which the skis are slotted.