When chlorine is added to water, a reaction occurs splitting it into hypochlorous acid and hypochlorite ions. The hypochlorite ion does not actively contribute to the sanitizing ability of chlorine (only 1%), but is an extremely strong oxidizer The hypochlorous acid is the active, killing form of chlorine and what does the actual sanitizing. This chlorine molecule easily enters micro organisms through their cell walls and kills the organisms by destroying the sulfur groups on the cell's enzymes, causing the cell's metabolism to stop, resulting in the cell's death. Not only does chlorine kill bacteria, it does it very quickly.
The amount of hypochlorous acid and hypochlorite ion that is produced when chlorine is added to the water is directly related to the pH of the water. At a pH of 6.0, 96% of the chlorine will become the desired "killing" form of chlorine, hypochlorous acid. At a pH of 7.0, 75% of the chlorine will become hypochlorous acid and at 8.0 the hypochlorous acid concentration is only 25%. At a pH of 7.5 (the average spa) the amount of hypochlorous acid produced is about 50%.
The hypochlorous molecule will continue to "kill" until it combines with a nitrogen or ammonia compound to become a chloramine, or is broken down into its component atoms and becomes "neutralized". Chloramines do not poses any substantial sanitizing ability and are usually the cause of many unpleasant problems, including eye and mucous membrane irritation and the source of chlorine odors. "Freeavailable chlorine" at levels up to 10-20 ppm has no detectable taste or odor and causes no irritation.
When bromine is added to a spa it's in the form of bromide ions. It then activated with an oxidizer to form hypobromous acid. Hypobromous acid is the killing form of bromine. When hypobromous acid reacts with an organic contaminant it's reduced back to bromide ions. The bromide ions can then be reactivated back into hypobromous acid by the addition of an oxidizer.
Bromamines actually do continue sanitize and oxidize.
Nature 2 is an ion type system (think copper ionizer). ions have the ability to pierce the protective outer membrane of a cell and disrupt enzyme balance thereby killing the cell. Ions are not oxidizers so you still need some chlorine or other oxidizer (MPS) to oxidize (burn up) the microscopic dead little bodies to prevent the water from becomming cloudy.