Jubilance makes a difference during leveling from 1 to Master Quest of 70(maybe 71) and then becomes crap.
Sitting in town you gain experience per second based on the #(number you have) ... example, you have 7 Jubilance, so you gain 7 exp sitting in town... ouch.
Skilling, the + jubilance adds to the experience (action based) on top of the skill experience gain.
Catacomb and killing, the +jubilance adds a number to the experience gained from the monster killed based on how many actions it took... so if it took 5 actions to kill... you get 5x 7 (say you have +7 jubilance) to the monster killed, HOWEVER. +Exp gain from an Item or Items or an Event can increase it (speculation here)
Now, the main issue is one thing... +Jubilance does NOT Work after character level 70 skilling or sitting in town, it would be assumed that this also means killing monsters in catacomb or map runs @ level 71+
No one has researched this from previous updates because Jubilance is deemed shit to use.