At a standstill, direct-drive can actually help you stay upright, sometimes motionlessly. Like nuclear weapons however, it's a point of subtlety to maintain certain resources while rarely demonstrating them. Friends don't let friends trackstand in traffic. Focus Fixed-gear road riding will test your concentration as well. Lapse of focus will be penalized immediately.
Consequences are potentially dire.
A fixed-gear trainer is a Pedaling Instructor who will demand unqualified attention. Should you forget to 'keep up' with the crankarms, you may merely have a foot kicked over the top of the stroke. A less forgiving mentor might elect to vault you upward off the saddle. You could even be tossed forward over the bars. A fixed-gear trainer can be a brutish riding companion, but one who's a hell of a lot of fun.