Within Synchronous there's always a clock from Tx to RX or from Master to slaves .
this clock is used to extact the timing of the signal.
In Asynchronous there's no clock and so TX & RX both work on a pre defined Rate.
a preambel is used for syncing the Rx
it's possible to start at the predefined rate and change the rate within transmission
according to dialouge between th Tx & Rx