Salt free water conditioners are designed to reduce or eliminate the problems associated with hard water without the use of salt or chemicals. They don't remove the minerals (as the softeners do) but they CONDITION the water, placing the minerals in suspension so the water behaves as soft.
There are at least two types: one which requires the use of a media like “Filtersorb ®” used by brands like Charger ® and Pelican ®, or a cartridge as used by Nuvo H2O ®, both resulting in a rather expensive periodic maintenance.
The other type and most favored by users require no maintenance and uses a variable electromagnetic field originally invented and patented by Scalewatcher® and currently used also by EasyWater®, Small Wonder® and others.
This method, also know as Physical Water Treatment or PWT works by inducing variable electric fields generated by a coil wrapped around the pipe. This forces the dissolved minerals such as Calcium and Magnesium to crystallize and remain in suspension in the water instead of adhering to walls of piping, faucets or heating elements of water heaters. The hard minerals now in suspension flow with the water and are discharged through the drain. This action stops any build up of hard scale. The capability of the water to dissolve minerals is also increased and this is the reason why pre existing hard scale layers will be softened, will slowly enter into the water stream, and will disappear down the drain. This effect will clean up all your piping, water heating element, including showerheads and faucets.
Independent research proving the effectiveness of this technology can be found in this page http://aquagenesisusa.com/en/technical/independent-research.html
There is a variation from this technology used by Hydroflow® or Hydrocare ® which uses a wave length closer to radio waves and which has, in many cases, also shown to produce the results advertised.