Servo synch is taking channels that perform like functions and transmitting them at the same time.

An example is dual elevators. With a 72mhz Futaba system the channels are polled and updated in order and if your elevators are on 1 and 6 then channel 1 gets its update, 2, 3, 4, 5 then the other elevator servo gets its position update.

This creates a noticable lag.

With 2.4 systems I can tell a noticable decrease in control lag when flying helicopters with high performance servos, on a typical airplane I cant, on a 3D gasser with good servos I can.