in my enemy turn what causes a Closed State that proveids a hidden card to react?

A Closed State exists whenever a Chain is on the stack — so on your opponent's turn, any Chain they open gives you a window to flip and play a Hidden card (it gains Reaction while facedown, Rule 811.6).

Chains only open when your opponent plays a spell, a triggered ability fires (e.g. "When you play me", Deathknell, Temporary banishment), or they activate an ability. That's it — phase changes alone never give you priority on their turn, and playing a plain unit/gear gives no reaction window because permanents leave the Chain immediately at finalization.

Two other notes: the Hidden card must have been hidden on an earlier turn (same-turn hiding can't be reacted with), and during a Showdown, if focus is passed to you, you can also flip Hidden cards even though that's a Showdown Open state, not a Closed one (FAQ #1855).