Model
Result
Drape side
Flip switches which of the two surfaces under your line gets measured — the near one or the far one. The string is then pulled against whichever side of that face makes it taut, so it always wraps something instead of collapsing onto the straight line.
After the first point, a protractor follows the cursor with the live angle and distance. It reads in the view plane — square the view up to the feature first, the same as laying one on a drawing.
Snapping
Click anywhere on the flat face around a hole, not the void — there is no surface in the middle of a hole to land on. It finds the bore, fits a circle to it and puts the point on the axis.
How it measures
The string is pulled taut: it rides the crowns and bridges every slot and hollow between them. That is also what keeps the reading honest — following the surface literally diverges, growing without limit the finer you sample it, because arc length only ever adds. Nothing under the string can reach the result.
Flip reads the other of the two surfaces under your line. For each one the string is pulled against whichever side makes it taut, so both give a real arc rather than one of them collapsing onto the straight line.