Is it in print that the complete cue ball must be behind the headstring? Some people think it's only the bottom of the ball that must be behind.
So, this is an interesting question that I had never really considered before. Anecdotally, from several years in various pool leagues, I can see that I've never seen anything enforced besides that the ball must be "placed" behind the head string, meaning the point of contact between ball and table is behind the head string. The rest of the ball may surpass the head string as long as "placement" is behind. Granted, that is a very small margin.
To be sure, I looked up the APA, UPA, and WPA rules on breaking and break fouls.
APA rules section 3 - "Breaking" - states that to be a legal break "players must break from behind the head string." Not much help there.
WPA rules section 6.10 - "Bad Cue Ball Placement" - states that "When the cue ball is in hand and restricted to the area behind the head string, it is a foul to play the cue ball from on or below the head string. If the shooter is uncertain whether the cue ball has been placed behind the head string, he may ask the referee for a determination."
I think the key word here, again, is "placed" behind the head string.
Lastly, UPA rules section 3.0 - "Legal Break Shot" - are the clearest of them all, stating "For the break shot to be legal, the breaker (with the base of the cue ball placed anywhere behind the head string)".
This clearly states the base of the ball must be placed, and only references the base.
From these, and my experience, I would say only the base of the ball must be behind the head string when playing from the kitchen.