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2 Added example
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Flash allows the card that has flash to be played at any time you could play an instant. It does not allow you to play any other cards at any time you have an instant. So Flash doesn't mean anything once it's on the battlefield. The actual rule:

702.8a Flash is a static ability that functions in any zone from which you could play the card it’s on. "Flash" means "You may play this card any time you could cast an instant."

You can summon a creature that has flash during your opponent's turn; just like you can cast an instant during your opponent's turn.

For example, you could wait for your opponent to attack you with his 2/2, cast Nephalia Seakite at the end of his Declare Attackers Step (on his turn), then block the attacker with the Seakite to destroy the attacker. Only the Seakite can be cast as an instant (unless you have another card with Flash), and once the Seakite is on the battlefield, its Flash has no effect.

Flash allows the card that has flash to be played at any time you could play an instant. It does not allow you to play any other cards at any time you have an instant. So Flash doesn't mean anything once it's on the battlefield. The actual rule:

702.8a Flash is a static ability that functions in any zone from which you could play the card it’s on. "Flash" means "You may play this card any time you could cast an instant."

You can summon a creature that has flash during your opponent's turn; just like you can cast an instant during your opponent's turn.

Flash allows the card that has flash to be played at any time you could play an instant. It does not allow you to play any other cards at any time you have an instant. So Flash doesn't mean anything once it's on the battlefield. The actual rule:

702.8a Flash is a static ability that functions in any zone from which you could play the card it’s on. "Flash" means "You may play this card any time you could cast an instant."

You can summon a creature that has flash during your opponent's turn; just like you can cast an instant during your opponent's turn.

For example, you could wait for your opponent to attack you with his 2/2, cast Nephalia Seakite at the end of his Declare Attackers Step (on his turn), then block the attacker with the Seakite to destroy the attacker. Only the Seakite can be cast as an instant (unless you have another card with Flash), and once the Seakite is on the battlefield, its Flash has no effect.

1
source | link

Flash allows the card that has flash to be played at any time you could play an instant. It does not allow you to play any other cards at any time you have an instant. So Flash doesn't mean anything once it's on the battlefield. The actual rule:

702.8a Flash is a static ability that functions in any zone from which you could play the card it’s on. "Flash" means "You may play this card any time you could cast an instant."

You can summon a creature that has flash during your opponent's turn; just like you can cast an instant during your opponent's turn.