From what I gather most dechlorinators use some type of thiosulfate (S2O3--)as an active ingredient. Thiosulfate has two extra electrons to donate so it's considered a reducing agent. If Chlorine (Cl2) is in the water it will accept the electrons and become the negative ion Chloride (Cl-) which is harmless. I found this typical reaction equation:
S2O3-- + 4Cl2 + 5H2O => 2SO42-- + 8Cl- + 10H+
If the chlorinating agent used was bleach (Sodium Hypochlorite, NaClO) there is a similar reduction of the hypochlorite (ClO-) to chloride.
I'm not sure about the reaction equation if the chlorinating agent is Chloramine (NH2Cl) but I believe that there is a similar reduction to chloride as well as ammonia residual production. So thiosulfates will neutralize the Chlorine portion but cannot neutralize the NH3 that forms.
Products like Amquel use a different chemical. An Amquel site identifies this chemical as Sodium Hydroxymethanesulfonate, HOCH2SO3Na. According to that webpage (http://aquabaz.tripod.com/amquel1.htm) the hydroxymenthane part of the molecule (HOCH2) reacts with the ammonia to neutralize it.