[thstone]

Assuming that you have a Tiptronic transmission, the final click that you are noticing when pressing the accelerator to the floor is the "kick-down" feature.

This feature is engaged when the accelerator is pressed beyond full-throttle. In this mode, the transmission will automatically shift to a lower gear to provide maximum acceleration.

Once kickdown is engaged, the transmission will only shift up at the maximum engine speed. This mode is disengaged when the accelerator is released to approximately 80% of full throttle.

For further reference, see page 142 of your owners manual.