On a Counter-Rotator a "Crack-Roll" is being able to roll the heli 90' then snap it back to vertical without CRASHING!
Blade clearance between the upper and lower rotors is one factor, the other is the weight of the flybar.

More weight=more stability (less reactive to cyclic input), less weight=less stability (faster reaction to cyclic input).

The centrifical force on the flybar with the attached weights, slow the upper head drastically when you push the BIG LAMA into a hard banked turn, once the upper rotor looses headspeed, it yaws (from uneven torque), stalls and plummets to good 'ol Mother Earth.[:'(] Notice the lower rotor has no flybar or additional weights, ever notice when you throttle-down the lower rotor stops 1st & the upper keeps going do to the CF. Weighted blade tips on 700mm & larger blades have the same effect with CF.

I tried alot of these mods already on a Hirobo Shuttle XRB, homemade CF swashplate, flybar paddles. But it's useless outside in any breeze, and the foam blades have there limits as to how fast you can push them, before they snap off at the blade holder.

Shadow