Auto-exposure

In short: Auto-exposure stretches the colour gradient to the terrain currently in view, so even very subtle, local elevation differences become clearly visible.

Spreads the colour ramp across only the elevations currently in view: the lowest visible point takes the bottom of the ramp, and the highest the top. This preserves contrast even in very flat terrain — over the Netherlands, for example, just 10 m of relief can span the full ramp instead of being compressed into a tiny sliver by a fixed scale. The trade-off is that a colour stops meaning a fixed height.

There are two caveats. First, in a tilted view the frame can extend all the way to the horizon, so either end of the ramp may be set by terrain far from the area you are looking at — or even by terrain that is not actually visible: the bottom of the ramp may be determined by a valley floor hidden behind the peaks. Second, the elevation range is only an approximation of the visible frame. It is read from elevation tiles in coarse blocks, and any block touched by the frame counts in full, so terrain just outside the view can affect the result. The discrepancy is typically tens to hundreds of metres when looking straight down in the mountains (over flat country it shrinks to metres), and can grow in tilted views because the frame covers a much larger area of terrain and uses coarser elevation tiles for distant ground, making the estimated range less precise.

For more details read the docs. The idea comes from Heightmapper.