Shown below is a near-miss encounter in the opposite direction of the galaxy disk spin.
Very little happens. Now notice the same encounter in the direction of the galaxy's spin or in corotation with the orbit of the pair.
Now each of the outer particles, rotating in the direction of the encounter, experience the impulse of the encounter for a longer time, and energy is added in the direction of their natural orbital motion.
A significant amount of material is liberated in the form of
Key to the process was that orbital energy between the galaxies was transfered into internal motion of the stars as shown in this close-up of the outer ring of particles.
These types of simulations were applied to numerous objects in the sky with the growing belief that the only variables to be solved were the orbital geometry of the interaction (the mass and size of the galaxies could be measured), such as the Antenna Galaxy below.