Can you reveal a hidden card with an hourglass (temporary) unit in reaction to the temporary trigger at the start of your turn, or does the unit die and you lose the hidden card before you can reveal it?

Ruling: You can reveal the hidden card in reaction to the temporary trigger. The temporary keyword is a trigger, not a passive effect, so you have the opportunity to respond to it before the unit dies.

Sequence:

• The temporary trigger occurs at the start of your turn
• You can reveal your hidden card in reaction to this trigger
• If you don't react to the temporary trigger, the unit will disappear and you lose the hidden card

Nuances:

• This only works if your unit dies in a showdown (combat or otherwise), because control can't be lost for the duration of any showdown, giving you a window after the unit dies and before the showdown ends to play your hourglass
• Outside of a showdown, control will be lost in the cleanup following the unit leaving the battlefield and you'll lose your hidden card, so you'd need to respond to the effect that removes the unit