As andrew says, it was a bit horrid!
Th idea was to produce special layers, which held only "cutting" lines. A "cutting" line was drawn across the object (e.g. the stem of a bolt) on this layer and the stretch took place at this point.
The layer name had to follow a fixed format e.g. "_stretch_lr_5". This means that items will be stretched (always at right-angles to the "cutting" line), the "l" means that you will be prompted for the length, the "r" means that the stretch will be to the right and the "5" means that existing length is 5 and that, if the user asks for, say, a length of 16 then 11 will be added.
I can see how this could quickly result in a load of unrecognizable layers - almost impossible to sort out if many objects are to be stretchable.