To convert selection from TrueType to PostScript, use the Contour > Convert > To PS Curves menu command. Conversion of curves from TrueType to PostScript can be “perfect” (lossless), using all the TrueType nodes and adding more as needed. However, the setting at Preferences > Curve Conversion allows you to control how much approximation is allowed, and a slightly liberal setting can make for a conversion with far fewer points:
To convert selection from PostScript to TrueType, use the Contour > Convert > To TT Curves menu command. Conversion of curves from PostScript to TrueType is always something of an approximation since the Bézier curves used to describe Postscript outlines are more complex than those used for TrueType outlines. Again, the tradeoff between accuracy and keeping a minimal number of points can be controlled at Preferences > Curve Conversion.
In the Glyph window, if no selection is made and the operation is run, all the outlines in that glyph will be converted. Note that while you can make a selection within a glyph, you cannot convert only a segment of an outline. If you select a segment and run the operation, the entire contour to which it belongs will be converted.
But there is a method to convert a segment to TrueType/PostScript curve: simply Alt-click on the segment. In order to use this feature, the option Clicking on outline selects segment must be off. Otherwise Alt-clicking selects the whole contour.
In the Font window, all the selected glyphs will be converted if you run the operation. Select several glyphs at the same time by clicking on the glyphs of your choice while holding down Cmd, or if they are in a continuous range, by clicking on the first and last glyph of the range while holding down Shift.