Found it! 'He held tightly onto his partner and had her do a headfirst somersault.'
I knew I'd read this description of a tango event in a well-researched history of tango, and it's taken me a while to re-find it. The question is: did this event occur in a) 1948: b) 1997: c) 2009?
Bonus points for answering where: a) on the set of 'The Tango Lesson': b) Club Nelson; c) Cachirulo milonga?