The constant excuse for saying anything shows some uncertainty regarding what instantaneous awakening is. However, Chan teachers, starting with Heze Shenhui, were quite clear about it that non-thought is all there is to realise. That there is nothing that could be grasped/attained, everything is originally empty, the mind is pure from the beginning - these were all known teachings well before the emergence of Chan. The difference between gradual and instantaneous lies in how the gradual path gives a step by step instruction to how to reach non-attachment, while the direct path is just not attaching to anything. That's how there is actually no such thing as sudden enlightenment, as it all depends on the individual's qualities. Either one understands it immediately or not. And even if there is a clear realisation that the six sensory impressions are insubstantial, it is easy to fall back to one's habitual clinging to phenomena, thus many Chan teachers emphasised continuous training, or the so called "sudden enlightenment, gradual practice" format.
So, how is there instantaneous awakening? One only needs to observe the emerging and disappearing phenomena of sights, sounds, feelings and thoughts to confirm that there is nothing anywhere in one's realm of experience that stays even for a moment, so there is nothing to hold on to or reject. Why is this information/instruction useless most of the time? Because knowing how to observe is already a technique one learns through calming the mind and detaching from constant conceptualisation. Because even if one can personally confirm that all sorts of identification and clinging are false and mistaken, that attachment is the true source of dissatisfaction and suffering, as a result of habits one easily forgets about all that.
"If the Buddha-Nature is seen, there will be no seeing of a nature in any thing."
(Nirvana Sutra, T12n374p521b3; tr. Yamamoto)
"Neither cultivation nor seated meditation — this is the pure Chan of Tathagata."
(Mazu Daoyi, X1321p3b23; tr. Jinhua Jia)
"With sudden enlightenment to Tathagata Chan,
The six paramitas and myriad means are complete within that essence."
(Yongjia Xuanjue, T2014p395c14; tr. from "The Sword of Wisdom")