Do cards with hidden only gain reaction speed when they are played facedown or can they also be played at reaction speed from hand?

Cards with the Hidden keyword can only be played at Reaction speed after they have been hidden facedown on a battlefield. They cannot be played at Reaction speed directly from your hand.

According to Rule 811.1.b, the Hidden keyword functions as follows:

"While this card is in your hand or in your Champion Zone on your turn during an Open State, you may pay [A] to hide this facedown at a battlefield you control... Beginning on the next turn, this gains [Reaction] and you may play this, ignoring its base cost."

To summarize:

• From your hand: The card must first be played using the "Hide" action (which costs [A]) to place it facedown on a battlefield. This action is performed during an Open State on your turn.
• From the battlefield (facedown): Beginning on the next turn after it was hidden, the card gains the Reaction keyword, which allows you to play it at Reaction speed.

Essentially, Hidden is a setup mechanic that requires you to place the card on the board before it gains the ability to be played at Reaction speed on a later turn.