To flip-flop; to do an about-face — suddenly change one's attitude or opinion, often reversing a previous stance.
当初は応援していたのに、彼は選挙直前に手のひらを返した。
Although he had supported him at first, he flipped-flopped just before the election.
あの上司は褒めていたのに、ミスがあるとすぐ手のひらを返す。
That boss praised you, but as soon as there's a mistake he immediately turns on you.
世論は手のひらを返すように対象を批判し始めた。
Public opinion began criticizing the subject as if it had suddenly turned.
To do something easily; to accomplish a task quickly and effortlessly (used figuratively).
彼は経験があるので、その書類整理は手のひらを返すように終わらせた。
Because he had experience, he finished organizing those documents in no time.
プロに頼めば、面倒な手続きも手のひらを返すように済むだろう。
If you ask a pro, troublesome procedures will probably be handled easily.