"If you must play, decide on three things at the start: the rules of the game, the stakes, and the quitting time.” -- Chinese Proverb
Darby and Bonnie seem to have no trouble deciding on all three things.
The rules of the game seem (to me) to be "anything goes." Bonnie prefers grabbing Darby by the neck; Darby usually shoulders her way up to Bonnie and bites at her legs (or cankles!) but, neither girl is above nipping at any body part that gets in the way.
The stakes are never high. They both know the prize is fun and neither seems to want it at the expense of the other.
And, the quitting time? Well, that is the one thing they often disagree about! Bonnie usually has had enough long before Darby wants to stop and she doesn't hesitate to let her quitting time be known!