When Volibear (Furious) attacks and triggers his ability to deal 5 damage distributed among units, can the defending player play hidden units in response and have those units become valid targets for the damage distribution?

Ruling: No, when Volibear's trigger is put on the chain, targets for the damage distribution must be chosen immediately and are locked in. By the time the defending player can respond to the trigger, the targets have already been selected.

Sequence:

• Volibear attacks and his trigger goes on the chain
• The attacking player chooses targets to distribute the 5 damage among
• Targets are now locked in
• Priority passes to the defending player, who can now play hidden cards
• The trigger resolves with the previously chosen targets

Nuances:

• Hidden cards can be played at any time you have priority after the turn you hide them
• If a unit would die outside of a showdown (like from Falling Comet), you must respond to the effect before the unit dies to maintain battlefield control and prevent losing other hidden cards
• If you wait until after the unit is dead (outside of a showdown), it's too late to prevent losing battlefield control