Chloramines are formed when chlorine reacts with ammonia and some organics. In a low bather-load pool such as most residential pools, there is a very small amount of such ammonia and organics being introduced into a comparatively large amount of water. Chlorine will continue to oxidize most chloramines to final end products which is why you don't normally see a buildup of Combined Chlorine (CC). The UV in sunlight also helps by both directly breaking down some chloramines (dichloramine in particular) and indirectly by breaking down chlorine to produce very powerful but short-lived oxidizers called hydroxyl radicals. These hydroxyl radicals can oxidize some organic precursors before they get a chance to react with chlorine to form chloramines.
The only thing special about the TFP method that results in a more consistently lower CC is that a reasonable active chlorine level is maintained in the pool at all times. Accurate measurement and consistent dosing result in consistent disinfection and oxidation.
My own pool has a mostly opaque electric safety cover on it except when the pool is in use nearly every day for about an hour at which time at least part of the pool is exposed to sunlight. I consistently measure < 0.2 ppm CC (i.e. the limit of detection in the FAS-DPD chlorine test using a 25 ml sample size). It's most likely < 0.1 ppm CC and the primary component of CC is likely to be monochlorourea which is innocuous.
