Right after the discovery of QSOs, people have found the existence of a new class of QSOs, showing wide absorption troughs associated with the prominent resonance lines such as CIV, SiIV, Ly-alpha and
NV. These class of objects were later characterized as broad absorption line QSOs (BALQSOs). The broad absorption feature is usually caused by the series of absorbing clouds outflowing towards the observers with velocities reaching up to 0.1c. The nature of acceleration mechanism driving these strong outflows in BALQSOs is still debated. However, it is well established that radiative acceleration plays an important role in the kinematics of the absorbing clouds. In this talk, I will discuss observational signatures of these outflows supporting the key role of radiative acceleration.