When autosave starts there is a brief rewind of the onscreen action while riding a boat.
I.e. if riding a boat the action moves backwards briefly.
And a boat will splash water when moved back.
While riding a minecart the action is paused momentarily as the autosave begins then the cart dashes forward and returns to normal speed.
Also if you are placing a block as autostart begins there is a delay before the block appears where you placed it.
Breaking blocks, particles appear, but the block remains in place then breaks.
When fighting mobs, the mob will freeze for a moment (slimes, blazes freeze in mid-air) then race forwards. Like they freeze then perform a rush attack.
Kind of funny seeing rabbits freeze in mid-hop.