That is a term used to describe the permanent distortion that a torque converter housing can experience if it is not strong enough to withstand the internal forces it's subject to. The overall length of the converter will grow, taking up the much needed freeplay that it should have which keeps it from bottoming out in the transmission pump.
One means of combatting ballooning is by welding support plates to the converter cover and impeller housings for added rigidity. Another is manufacturing a purpose-built, high-performance converter cover that can withstand greater stress than its OEM counterpart.
