Over on another popular forum for faces that has a 500 Racing page dedicated to 500 F3 a poster understands that "thropping" is the act of leaning into the corner, and reaching down to hold on to the side of the car as demonstrated above by Andrew Turner in his Cooper Mk 6 at last years Castle Combe Autumn Classic.
He wonders if anyone else has heard the term or if Pete Becker was pulling his leg ?