First I should note that in 8 ball pool, "ball-in-hand from behind the head-string" is not a thing according to the World Pool-Billiard Association 8 Ball rules. These WPA eight ball rules are the predominant rules for 8 ball, and they state that if you commit a standard foul, then play passes to your opponent with "ball-in-hand anywhere on the table".
But you are right, some localized leagues (e.g. BCA) have rules for this, and thus, override WPA rules when playing in those respective leagues - but I don't think it's as common.
I'm going to assume that you are playing by the BCA's eight ball rules (and by extension, the BCA's general rules of pocket billiards - (playing from behind the string) rule:
General Billiard Rules - Playing From Behind The String (Regulation 3.39)
When a player has the cue ball in hand behind the head string, "in the kitchen", he or she must drive the cue ball to a point across the head string before it contacts either a cushion, an object ball, or returns to the kitchen.
Failure to do so is a foul if a referee is presiding over a match. If no referee is present, the opponent has the option to call it either a foul or to require the offending player to replay the shot again with the balls restored to their positions prior to the shot with no foul penalty imposed.
An exception exists here: if an object ball lies on or outside the head string, and is thus playable, but so close that the cue ball contacts it before the cue ball is out of the kitchen, the ball can be legally played, and will be considered to have crossed the head string.
If, with cue ball in hand behind the head-string and while the shooter is attempting a legitimate shot, the cue ball accidentally hits a ball behind the head string, and the cue ball crosses the line, it is a foul.
If with cue ball in hand behind the head string, the shooter causes the cue ball to hit an object ball accidentally, and the cue ball does not cross the head-string, then the incoming player has the option of either:
- calling a foul and having cue ball in hand, or
- having the balls returned to their original position, and having the offending player replay the shot.
If a player under the same conditions intentionally causes the cue ball to contact an object ball behind the head-string, it is considered unsportsmanlike conduct.
In this case your only option (if you decide to play your shot) is to send the cue ball up-table, bank it off the opposite end rail sending it back to (hopefully) strike your only remaining object ball which was so unfortunately placed.
It really is a rule that perhaps should be re-examined, in my opinion, since it basically punishes you when it should be rewarding you for your opponent's mistake.