Agitation of the developer is important for bringing fresh developer uniformly to the film and carrying away exhausted developer. With insufficient agitation, several kinds of unevenness can be present in the final image.
In an inversion-tank, you depend on the large bubble of air in the tank lid to move throughout the tank when inverted. The bubble goes up through the spiral and the developer drains down into the lid, without the bubble and the ability for the developer to flow around the bubble, there is no effective agitation. Therefore, you must not overfill the tank; stainless tanks are quite susceptible to overfilling.
You want to use only just enough developer to cover the film if possible. More developer should be used only if required for the chosen dilution and in no case should the listed capacity of the tank be exceeded.
On 35mm film, poor agitation often presents as faint streaks across the image that line up with the sprocket holes; these are known as surge marks due to the local turbulence caused by the holes. If proper agitation were occurring, this small bit of extra turbulence would have no effect.
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