I found out about this shortcut from the Excel tips newsletter
This mouse shortcut can be used to navigate around your worksheet. When you select a cell, Excel places a bold outline around that cell. If you double-click on one of the borders of the cell, Excel moves the cell selection in the direction indicated by the border you double-clicked. No data is moved, it is a way to quickly move to another part of the worksheet.
If you are in the middle of a data table then the cell selection will move to the edge of the data table. For example, double-click on the bottom border and it moves the cell selection to the bottom of the table. If you are in an empty cell, double-clicking on the border will change the selected cell to the first cell adjacent to a cell with data in it, again in the direction indicated by the border of the cell. If there is no data in cells to the right or below the selected cell then the selected cell will not change.
When the selected cell is inside a data table, this mouse shortcut works in a similar way to using "Control + Directional arrow" on the keyboard, but behaves differently when in an empty cell.