Flash permits the card with flash becoming played anytime you can play an instant. It will not enable you to play other cards at any time you've got an instant. Therefore Flash does not mean something once it's regarding the battleground. The actual rule:
702.8a Flash is a fixed ability that functions in any zone from which you might have fun with the card it's on. "Flash" implies "You may play this card when you could throw an immediate."
You can easily summon a creature that has flash throughout your adversary's turn; just like you can cast an instant through your opponent's change.
For instance, you might watch for your adversary to attack you with his 2/2, cast Nephalia Seakite at the end of his Declare Attackers action (on his turn), then block the attacker using the Seakite to destroy the assailant. Just the Seakite may be cast as an immediate (unless you have another card with Flash), as soon as the Seakite is on the battlefield, its Flash does not have any impact.