The key phenomenon at play here is the two-photon recoil process. When a cold atom in the BEC absorbs a photon from one of the laser beams, it gains energy and gets excited to a higher energy level. Almost instantaneously, the atom emits a photon in the opposite direction, in the other laser beam, and returns to its initial state. As a result of this two-photon process, the atom receives a net momentum kick with a magnitude of 2ℏ𝑘 (where ℏ is the reduced Planck's constant and 𝑘 is the magnitude of the wave vector of the laser) in the direction of the absorbed photon.