I experienced it when I went through a binge-period with LSD many many years ago, I remember it been a sense of extreme detachment.
Imagine speaking but not been able to identify with what your saying or even the sound of your voice, there's a disconnect between self and reality. To me it makes sense that this would result from heavy psychedelic use as psychedelics often reveal the identity to be a construct of the mind, but as you return to baseline so does your sense of self.. but with continued use you remain in that haze of self-dissolution. I've had moments of it with increased anxiety brought on by stress completely independent of psychedelics, like for example: speaking in front of a group of people.. due to anxiety I will suddenly detach from what I'm saying and almost observe myself speaking in third person which cause's me to lose my train of thought, which then snowballs into greater anxiety.
But with psychedelic depersonalization it feels like a consistent existential detachment, but it's been so long that I can't really remember the specific details to describe it.