It's an old technique.
Suppose your source is currently:
SELECT FieldA, FieldB, FieldC FROM TableName
with a column widths, say:
1; 3; 3
But you want FieldB to be displayed upon selection then you could change the source to
SELECT FieldB, FieldA, FieldB, FieldC FROM TableName
and up your column count to 4 (of course) and set your columns to:
0.002; 1; 3; 3
That way the first column won't be rendered in your dropped down list (it's too narrow).
However, as the first non-zero width column listed, it will be displayed upon selection of the value.
Cheers.