What is wing trammelling?
It's just making sure the rectangle formed by the spars, compression tubes and cross bracing is square.  It's done using a trammel bar, which is just a stick (which the guy in the picture is holding) with a metal point at each end that is used to make sure diagonal reference points along the spar beams are the same distance apart when set across one leg of the X formed by the cross bracing, then the other leg.  The effect on stability and performance is indirect insofar as you don't want a rectangular wing to be trapezoidal (swept - unless the sweep is part of the design like the upper wings of some biplanes, in which case you trammel to that requirement), as well as the fact that parts won't fit if things aren't square.