...... the disinfectant is still chlorine (as hypochlorous acid HClO), and it this trickles out as the chloramine breaks down to Cl- and NH3(NH4+). Because it is trickling out over time, a planted tank will mop up the NH3.
Seachem won't tell you how "Prime" works, but Kordon's "Amquel" has a patent
<http://www.kordon.com/kordon/products/water-conditioner/amquel#compatabilities!> and I would be very surprised if the mode of action of Prime is much different.
I would suspect that all the newer, "better" conditioners (have a look at the blurb for "Amquel+") use EDTA and sodium hydroxymethanesulphonate (or similar). The EDTA chelates any heavy metals (as long as Fe ions aren't present in large amounts) and the hydroxymethane - end of the molecule reacts with ammonia to form a non-toxic, stable water-soluble compound "aminomethanesulphonate". The sulphonate end of the molecule reacts with both free-available chlorine, and combined-available chlorine in chloramines. Any ammonia (from the break down of the chloramine) is then mopped up by the sodium hydroxymethanesulphonate.