below the belt=unfairly

He pretended to be her friend, then applied for her job. That was really below the belt.

below= farther along than

There is a picnic ground just below the bridge

behind the scenes= not seen

He had a lot of help behind the scenes.

behind the times=  old fashioned

Her dad still uses a typewriter; he is really behind the times.

get back at some one= do harm in return for a wrong

After he was fired, he tried to get back at his boss.