When this option is disabled, and you don't already have the Unreal Engine genlocked to an input signal, the engine runs at the fastest frame rate it can manage and provides all the frames it generates to the AJA card. Each time the card is ready to output a new frame, it selects one of the frames generated by the Engine. When this option is enabled, the Unreal Engine does not generate any new frames of output until the AJA card is ready to accept the new frame. The effect is similar to genlock, but instead of locking the Unreal Engine's frame rate to an input signal, it locks the Engine's frame rate to the output timing of the AJA card. This option is most useful when you don't already have an input signal that you can lock the Unreal Engine's frame rate to, but you want to ensure that the Engine is producing only one output frame for every frame in the output video feed.