"Don’t allow kittens to bite or scratch when playing. This causes aggressive behavior."
This simply means do not let the kitten treat your hand as just another play-toy. It may be fun to play 'snake' and let the kitten leap and attack your hand and forearm ... BUT ... it is cute when they are little and not-so-cute when they are bigger and their teeth/claws can really hurt and do some damage. They won't understand they are hurting us because we *taught* them that playing in that manner was acceptable.
When you first begin to rub a kitten's belly, depending on age, it can be seen as a signal to play and/or it tickles so the kitten will instinctively try to kick your hand away or claw/bite it. If you want to get it accustomed to having its' belly touched, just move your hand slowly and if it bites/grabs, stop moving your hand and say "No". When the kitten stops, continue moving your hand. You could also use your other hand to gently disengage the kitten from your hand.
