@ Carl, WM: sodium bromide is converted to free bromine when you add a chlorine source, such as bleach. Ammonium chloride is the other "Yellow X" product; it combines with chlorine to form monochloramine. You have to destroy monochloramine before swimming, but you can swim in brominated pools just fine. The problem is, you cannot 'stabilize' the chlorine (bromine!) loss, so usage is very high.

@ All: the mustard treat thread is here: http://www.poolforum.com/pf2/showthread.php/25757

"Stop Yellow" appears to be a Pinch-a-Penny product of the ammonia variety.