For what it’s worth, in the novelization, Poe sets a hyperspace entry point before Leia and Holdo retake command of the ship. That entry point is never changed, so when Holdo is alone on the ship waiting to die, she notices it - and because the ships have kept moving that whole time, the entry point is now behind the First Order fleet. That’s what gives her the idea to try it - because the ship will be accelerating to near lightspeed prior to hitting the hyperspace entry point.
So the “hyperspace ramming” actually occurs before the ship enters hyperspace, while the ship is accelerating to near-lightspeed in realspace.
It was basically her seeing where the entry point was plotted in relation to the position of the fleet, getting the idea, and going “Huh, wonder if this will work? Can’t hurt to try since I’m dead at this point anyway.”
Huh, that’s a fairly good in-universe explanation. So it’s not the hyperspace travel itself that makes the collision but the “flicker of pseudomotion”, as Zahn puts it.
EDIT: That could also go some way towards addressing my biggest problem with that scene - why no one thought to do that maneuver before losing 90% of their fleet. They couldn’t until the entry point was sufficiently far away behind the First Order.
Even Zahn states with the ‘pseudomotion’ that the ship isn’t really accelerating to the speed of light, otherwise it would be, you know, actual motion.
Hyperspace is a way of getting around the infinite energy required to accelerate a ship to the speed of light. Requiring ships to accelerate to light speed to enter it defeats the purpose.