On Thursday, April 30, 2015 at 10:16:16 AM UTC-4, wrote:

There is nothing fundamentally dangerous about the technique when properly done.


The BGA reference (page 30) says that pulling the tug laterally has the potential to abruptly stall the vertical stabilizer of the tug, inducing a flick roll of the tug and a possible midair collision. It goes on to say "The highest risk of a lateral upset is during the
'glider cannot release' signal demonstration".


See Section 42 of the BGA Aerotowing Guidance Notes (LATERAL
TUG UPSETS):

https://www.gliding.co.uk/bgainfo/cl.../documents/aer
otownotes.pdf