Scratching at fading memories here but isn't the range of the main electric trim on the 737 less than the full range of manual trim wheel inputs, hence full nose down trim will be unrecoverable without manual trim wheel input?
Actually, depends on flap setting, if flaps are in a different position than 0 we can use fast speed electric main trim for nearly the whole trim range (forward limit is 0.4 units), if flaps are 0 forward trim range is limited. Even if manually set full forward you can allways use the electric main trim to trim backwards.