[quote=@Scarifar]
<Snipped quote by Ryuzaki>

I think that right there was what lost me. What I said in my summary of their characteristics, though, explains it a bit better. Imo, at least.
[/quote]

Not quite. When it loses its overlays, it's still in the game. The overshift happens when you try to detach something that isn't there.

Edit: Well, more specifically, when you try to activate an effect without the overlays. A phase shift monster can have its effect as long as it has overlays, but the moment those are used up the effect is negated and they retain the attack and defense of the last phase. Once you can activate one of their effects, gain the appropriate phase, and at the end of the turn you did that it is banished.