Prowess gives Jeskai’s creatures additional +1/+1 boosts until the end of the turn whenever you cast a non-creature spell, so then giving them flying and double strike is extremely deadly. Still, it's a lot of mana to play at instant speed. However, it does untap all creatures you control and gives your creatures flying and double strike until end of turn.
A common question that's asked about Flying Crane Technique is if you can cast it even after you declare attacks. You can indeed declare attackers, then cast it, and get the full effect. However, if blockers are declared, any creatures that are declared blocked will still be blocked even if they gain flying. On the other hand, gaining double strike can affect the outcome against those blockers. Do keep in mind, however, that creatures that come into play after you cast this will not gain flying or double strike - only those creatures that you control as you cast it.
Without a doubt this is a finisher. If you happen to attack with Narset, Enlightened Master and exile it using her ability, you even get to cast it at Instant Speed for free. This card can easily end the game on its own, especially if cast for free.
One place this card can definitely be good is in Commander, where Narset decks have run it with some success. Other White/Blue/Red Commanders Ruhan of the Fomori and Shu Yun, the Silent Tempest may find a way to integrate it into some of their builds, as well.
While it hasn't seen any competitive play outside of Limited formats such as draft, Flying Crane Technique is most definitely a bomb that can serve as a useful combat trick that comes out of seemingly nowhere.