Record Video Out allows traffic optimization from GPU to CPU memory. Normally the output frame is transferred twice, once for the SDI output and once for the recording. If you turn this switch ON the transfer only happens once, because Recorder uses the already transferred frame of the SDI output.
If you work in HD you should not pay too much attention to this, simply turn it OFF, and forget about it.
If you work in 4K then it might have some advantage in certain high load cases.