There is no synchronization. The keyboard toggles the clock line, and circuitry inside the Amiga detects this long clock pulse and asserts _kbdreset.
If the machine doesn't do anything, the keyboard gets reset (the caps lock led blinks) and continues working as normal. Naturally in this case the computer probably won't respond to any of your keypresses, but the keyboard actually sends keycods as usual during this time.
The keyboard is not really synced in normal use either. It spits out a clock signal whenever it has data to send and the computer reads the data line according to the clock pulses. Each keycode transmission is a unique event.
If no-one answers the reset warning, the machine will reboot itself.
This is for big box machines. In the small keyboard style machines, the keyboard controller asserts _kbdreset itself, so the machine has no time to react before it gets reset.
