To launch the atoms, the frequency of the upward-directed lasers are shifted closer to resonance, increasing the photon scattering rate, while the downward-directed lasers are shifted further from resonance. This exerts a large upward force on the atoms.
The atoms accelerate until they reach a velocity where all of the lasers appear to have the same frequency because of the Doppler shift. This velocity depends on the frequency difference of the lasers and can be precisely controlled. The height of the launch depends on the launch velocity. Because of the large accelerations possible with laser manipulation, launch velocities of several meters per second can be achieved very quickly.
Our atoms are typically launched at a velocity of 4.4 meters/second and reach a height of 1 meter above the collection region. The lasers are shifted to their new frequencies for a total of about two milliseconds (thousandths of a second).