It’s very simple
Say you are BR I and you kill a higher BR enemy, instead of getting the normal 30 XP you get +5 per every BR above yours and -2 for every BR below yours.
Defeating players at BR III would mean:
BR I = 30-2-2 = 26 XP / KILL
BR II = 30-2 = 28 XP / KILL
BR III = 30 XP / KILL
BR IV = 30+5 = 35 XP / KILL
BR V = 30+5+5 = 40 XP / KILL
This could also give new players a slight headstart instantly playing against BR III