Usually, a scene is generated using multiple texture stages. Each texture stage applies another layer onto the scene. Another common name for this is "Multitexturing". Examples of such texture stages are the diffuse map (general look and color of the objects), lighting map (the effect of lights on the scene), normal maps (the "bumpiness" of objects) etc. Some games use 8 or more texture stages. Only one texture stage, the diffuse texture map, is always required, to give the objects their appearance.
Most of these texture stages are not receptive for the negative effects of a missing trilinear filter (see above for details about these effects). Usually, it's only the diffuse map, who suffers the most if no trilinear filter is applied.
If a game takes full control over the filtering, it can tell the driver exactly which filter should be applied to each texture stage. This is the best way, because no performance is wasted for using advanced filters for stages that don't benefit from them, while still resulting in the best image quality. That is also the reason why it's always better to use an in-game setting for the anisotropic filter, if the game offers it, and not the driver setting or nHancer.
If you force the anisotropic filter using the driver (or nHancer), there's no way to tell for which texture stages the filter should be applied to. So if you disable the Anisotropic Filter optimization, the driver applies a full anisotropic and trilinear filter for all texture stages.
If you enable this optimization, the full trilinear, anisotropic filter will only be applied to the first texture stage, which is used for the diffuse maps in most games. All other stages only receive a reduced, bilinear anisotropic filter. This is fine as long as a game is really using the first texture stage for the diffuse map, but there are some games, which use a different stage for the diffuse map. In these cases, this optimization will cause a very visible "bow wash" effect, just like it was explained above.