This function has not been fully integrated into the long-term Mathematica system, and is subject to change.
BoxBaselineShift is an option for AdjustmentBox that specifies how much the baseline of the box should be shifted relative to those of neighboring characters.
The placement of a selection can be fine-tuned using an AdjustmentBox. AdjustmentBox objects can typically be inserted and modified using Alt+LeftArrow, Alt+RightArrow, Alt+UpArrow, and Alt+DownArrow on Windows and Unix, and Ctrl+LeftArrow, Ctrl+RightArrow, Ctrl+UpArrow, and Ctrl+DownArrow on Macintosh. These keys move your current selection by one pixel at the current screen magnification.
With the setting BoxBaselineShift->k, the baseline of the adjustment box is shifted vertically by kx-heights.