Can you explain how hidden works. If I have a unit and a hidden card at a BF and during a showdown my unit dies, can I then flip my hidden card after it dies?

The Hidden keyword allows you to place a card facedown at a battlefield you control. Here is how it functions:

How Hidden Works

• Hiding: You can hide a card from your hand or Champion Zone during your turn in an Open State by paying [A] (any rune). You place it facedown at a battlefield you control (Rule 811.1.b).
• Properties: Beginning on the next turn after you hide it, the card gains the [Reaction] keyword (Rule 811.1.b, 811.6). This allows you to play the card as a reaction during a Closed State (when a chain exists) or an Open State.
• Playing from Hidden: When you play a card from hidden, you ignore its base cost. The card is played to the same battlefield where it was hidden (for permanents), and targeting for its play effects is generally restricted to that same battlefield (Rule 811.1.d).
• Constraint: You cannot control a battlefield if you have no units there. If you lose all your units at a battlefield, you lose control, and any hidden card there is immediately sent to the trash (FAQ #4628).

Can you flip the hidden card after your last unit dies?

Whether you can flip your hidden card depends on the timing and whether the showdown is still active.

1. If the showdown is still active: You do not lose control of the battlefield when your last unit dies, because the battlefield is "Contested" until the showdown is finished (FAQ #4628).

   • Yes, you can flip the hidden card to play it during the showdown as long as it has the [Reaction] property (i.e., it was hidden on a previous turn).

2. If the showdown has ended: If your last unit dies and the showdown has fully resolved (with no remaining triggers like Deathknell to keep the state active), you immediately lose control of the battlefield. The hidden card will be sent to the trash before you have an opportunity to flip it (FAQ #5973).

3. The "Same Turn" Rule: Even if you still have a unit on the battlefield, you cannot play a card from hidden on the same turn it was hidden, because it does not gain the [Reaction] property until the next turn (FAQ #9904).

Summary: You can only play the hidden card if you still have control of the battlefield (because a showdown is ongoing) and the card has already gained the [Reaction] property (it was hidden on a previous turn). If you lose control of the battlefield, the card is trashed immediately.