Depends on how low it is. Early symptoms would be slow shifts, especially when cold, then as the level got lower you'll see delayed and hard shifts, meaning you'll lose power, then it'll kick hard into gear. None of this is good for the tranny, btw. Its possibly the most complex component on the vehicle, and they are finicky. You'll also be overheating the fluid and the internal components. These will not repair themselves when you add fluid. The damage will be cumulative.
Eventually, though, it'll mitigate itself by simply never engaging.
Just wondering....why would you ask the question?