It could start and malfunction, or it could start and work for a moment until one of the valves contacts a piston. Or it couldn't work at all. If you're not sure if the synchronization is turned off, it's not a bad idea to turn the engine by hand to make sure it's spinning without interference. With engine synchronization turned off, your vehicle will malfunction or may not work at all.
The engine will start to stall, run abruptly and cause other major problems that will not only impair the vehicle's performance, but will also cause devastating damage to the engine.