Initially held up by a magnetic field, the atoms fall when the field is switched off. As they drop down, most of the atoms collide with photons emitted by a laser diode placed directly under the trap, which emits pulses of light in an upwards direction about once every 2 ms. When the atoms collide, they each receive a precise amount of momentum, which knocks them back up. These atoms then fall down again, only to collide with the next laser pulse.
They can make very accurate measurement of the gravitational acceleration, and supposedly, plan to increase its accuracy.