ive only dyno'd a few cars... but in my experience, when i strap them down (using dyno dynamics dyno) that this will not stop them from jumping off the dyno.. the front wheels being choc'd will more then likely .. the strapping down with snatch straps via the lower control arms to the traction bar only achieves better downforce on the rollers for more accurate figures and hopefully preventing wheelspin..
i get alot of people who get scared of thier cars jumping off the dyno and hitting the brick wall 8m infront of the dyno haha... does no one understand the theory of momentum ... a car has none when on the dyno... its not going far if it jumps off, especially when choc'd and the operator backing off.