Hiding cards and playing them from Hidden

Hidden turns a card into a trap. You pay to hide it facedown at a battlefield you control, and from the next turn on you can play it for free at Reaction speed. The rules question is almost always "where, and at what, may it point?"

Worked example. On your turn you pay one rainbow rune to hide Elnuk Herd, a Hidden unit, at a battlefield you control that has no facedown card (rule 811.1.b). Hiding does not open a chain, and it is not playing, so "when you play" triggers and play restrictions do not apply to it (rule 811.1.c.1 and rule 811.1.c.2). On your opponent's next turn the Herd has Reaction (rule 811.6). When they attack that battlefield you play it from facedown for no Energy, which does open a chain (rule 811.1.c.3). Elnuk Herd's play effect gives a friendly unit +3 Might (it may even pick itself), and because it is a play effect of a hidden permanent, that unit must be at the battlefield the Herd was played to (rule 811.1.d.3). Then the Herd recycles, since it was played from facedown.

Hiding. Hiding a card is placing it facedown at a battlefield you control, and it is a Discretionary Action, so it is done on your turn in an Open State (rule 422.1, rule 422.2 and rule 811.1.b). A card can sit facedown there only as long as you control the battlefield. If another player takes control of it, the hidden card is removed and put in its owner's trash during the next cleanup (rule 323.6.a). A facedown card that would change zones is revealed to all players (rule 422.4). The turn after it is hidden, whoever's turn that is, it gains Reaction, and that is not public information (rule 811.6 and rule 811.6.a). A facedown card is not a card with Hidden (rule 811.5.a).

Where a hidden card can point. A hidden permanent, gear included, must be played to the battlefield where it was hidden, if able (rule 811.1.d.1 and rule 811.1.d.1.a). Unattached gear there is recalled to base in the next cleanup (rule 459.1). A hidden spell's targets must be at the battlefield it was played from, unless its text explicitly asks for something that makes that impossible (rule 811.1.d.2), and a spell with no valid targets under that restriction can't be played from Hidden (rule 811.1.d and rule 355.6). The play effect of a hidden permanent chooses targets at the battlefield it was played to, with the same exception (rule 811.1.d.3). So Tideturner, which may choose a unit you control at another location, is free to choose one there. Each target is checked separately: Smoke and Mirrors asks for a unit you control and another unit you control at a different location, so the first unit is still restricted to the battlefield it was played from, while the second can be anywhere else (rule 811.1.d.4). If a hidden spell or play effect plays a unit, that unit goes to the same battlefield (rule 811.1.d.5). A play effect reads its source's information, so if the permanent has left the board by the time the effect picks targets, it has no legal target (rule 811.1.d.3.a). Everything else on a hidden card works as normal (rule 811.2).

From hand. A card with Hidden can be hidden from your hand or your Champion Zone (rule 811.1.b), or it can instead be played from hand for its cost, at its normal timing and with no restriction on targeting (rule 811.3).

Common mistakes. "I hid it, so I played it": hiding opens no chain and triggers nothing. "A hidden spell can target anything": its targets must be at the battlefield it was played from, unless the card itself asks for another location. "Hidden means I can play it this turn": the Reaction arrives the turn after.