Hm, i saw this working, but i don't actually understand it
Visualise PQ relaitve to an origin O
m is the midpoint vector so it joins to the middle of the thing
then if u wanna divide it in half again to get fourths the position vvector of m and p added together and div by 2 gives the new midpoint vector of the midpoint and p, so basically u divided the half into a half so thats one fourth and u do the same for the otehr side