Why do we “watch the mind from the side” and not directly? If we watch the mind directly, the mind will be startled and disturbed, unable to rest quietly. If we proceed to watch the mind directly as soon as it enters a state of no- self, we will destroy that state since the thought of watching the mind is itself a kind of distraction. When that happens, the original state of tranquility disappears. As for watching the mind from the side, the mind can remain at rest and, at the same time, bring into full play its ability to supervise; as soon as distraction sets in, it will know and pull it back in place.
~Depicted from THE FOUR SEALS OF DHARMA - The Practice of Realizing Emptiness