There is a small heigth difference, just a few centimeters (1-2), between the oar gates. For balance you want to also have this difference between your hands. This is why left is above right. To understand why dropping the wrist to much is bad just put left above right while having your wrist dropped a lot. Now notice the height difference between your hands, definitely more then the difference between the oar gates. Furthermore extensive dropping of the wrist of also quite intense and possible bad for the wrist. Next to this rolling the fingers is more relaxing and allows to tap down and create a clearance between the water better. Sincr if your wrist is down, your hand and oar handle is more up. The only downside of that is a bit harder, especially in bad conditions. I tend to row with a bit of both. Especially considering that flat wrist is not a neutral wrist! Neutral wrist has a small angle of the hand going up, just a small slope. Just row with this small angle in the wrist and try to roll the fingers by do an relaxing small extension of the middle hand keeping the top of the vingers over the oar handle.