Back phrsing is a type of synocpation but applied to an entire musical phrase.
When you back phrase, you play the melody notes as written but you play them later than you are supposed to.
This creates a dramatic tension as the listener is waiting for the melody bu tyou hold it back and then release the tension when you let forth a beat or so later than expected.
The more you back phrase, the faster you have to play those notes of the melody that you held back.
You also have the option of dropping some notes out of the held back melody; this is more common with heavy back phrasing, where you might not have enough space to throw in the entire melody.