Easy, Say you were to KO a level 40 with a level 50, and normally it would have given 80 exp (That was a total guess). to you lv 50. But you have a lv 20 who would have gotten 140 exp had he knocked it out, holding a exp. share, so your lv 50 gets 80 exp divided by 2 and your lv 20 gets 140 exp divided by 2.

I hope that isn't confusing.