I am a lil confused about the whole "rinsing your head out with cold water"?

so i hear that when u rinse ur hair out with cold water after conditioning it... it seals the cuticles and seals in the moisture from ur conditoner. but if it seals the cuticles... and i put leavin conditoner on my hair afterward.. wuld that even matter? if my hairs not guna recieve much moisture from the leave in becuuz my cuticles are closed? pleaseeee inform me!!! thanku