Photos from a 1907 Scientific American magazine and found at http://aacalibrary.tumblr.com/
a long and throrough explanation of the act by Fred D. Pfening, Jr. Bandwagon, Vol. 13, No. 3 (May-Jun), 1969, pp. 20-24 can be summed up with;
the "golden days" of loop the loop and leap the gap acts was the period from 1900 to about 1912. All of the large circuses of that time used an act of this type as the closing number sensation one or more seasons. Sometimes it was on a bicycle and even on roller skates, but mostly by one or more autos. More frequently than not the passenger in the auto acts at the turn of the century were women, usually being billed as from France.
There was generally a pervasive idea inside and outside Madison Square Garden that his first performance would be his last. In fact there were two ambulances from the city hospitals present, one containing six surgeons who were anxious to have the first opportunity of examining the prospective corpse, and one filled high with air cushions on which Diavolo was to be carried in state, with so much life as remained in him, to Bellevue.
But it was the John Robinson's Ten Enormous Shows Combined that introduced to American circus goers the "loop the loop auto", in 1904. The "Great Mephisto" (Horace Mohn) circled the loop in an auto. Mr. Mohn appears to actually be he first person to do this stunt with an American circus, verified after a full search of newspaper ads, heralds, couriers and programs. However the Great Floto show came in second that same season when they presented "Kiro, monster African baboon looping the loop in an auto". Both of these acts used riggings with a barrell loop, and the cars did not somersault in the air.
Mlle. Octavia Le Tour, according to the New York Evening Post of March 23, 1906, risked her life strapped in an automobile, dashed down a dizzy incline and was hurled after turning a complete somersault in space, upon a padded aid railed gang way twenty feet away from the end of the incline. A four page newspaper size reprint of the various reviews of the 1906 New York date played up "The Limit" as the big act of the show. However the fate of Mile. La Tour and her auto following the New York date is unknown as she is not listed in the road program nor in the 1906 route book.
For the 1908 season Barnum & Bailey imported yet another auto thrill act, with yet another embellishment. This one presented the "Sisters La Rague" with two cars, using a 90 foot runway.
image from http://www.karenandjay.com/trips/bige08/bige08_29.jpg
Billed as an aerial auto race, one car painted red dashed down the runway striking an obstruction that tossed it into space for a mid-air somersault, while the other car painted blue followed closely, passing smoothly over the dropped obstruction and jumping under the red car, in a somersaulting position, to a landing ramp, just ahead of the red car that had left the top of the ramp first. These two French gals were back for the 1909 tour, which was the last year that the Greatest Show on Earth used an auto or bicycle thrill act.
info article from http://www.circushistory.org/Bandwagon/bw-1969May.htm