Indeed, a lower threshold would be like trampling, a higher one charging/ramming. That's quite nice and flexible!

The damage is scaled with relative speed. And it has a threshold* you can set in the template (could be 0 too). It's called charge in the code but trampling could be somewhat descriptive too since it's any collision that would trigger the effects. *the threshold is only checking the charger own speed, so 0 for elephants makes the unit coming into contact to also count as hit. So it is somewhat a bit trampling. 

So, like centurions for boats? I'll consider this for my boats mod

It is shift-drag for me The grayed selection area is showing for my mouse, but nothing is showing in the list

I agree with all that @guerringuerrin said. Those tests look really good, and in such tight spaces. @Atrik, since I guess now with inertia units need to accelerate, is heavy cavalry dealing "charge" damage only when at max speed? (I'm not sure if this is what's called "trampling", which wouldn't be the same thing in reality). Because I guess that would be nice, one would need to repeatedly move out and smash for that to be more effective, similarly to how it should really work for ramming ships, although elephants I guess could charge out when mobbed.