From the manufacturer

Dynamic Gold AMT shafts have the same performance DNA as Dynamic Gold and are designed to get progressively heavier as golfers move from their long iron to short iron. Lighter long irons produce increased speed and higher initial launch, which makes them easier to swing and deliver a softer landing. The Tour-weighted short irons produce traditional Dynamic Gold accuracy, consistency and trajectory. AMT stands for Ascending Mass Technology, which means the shaft’s weight increases in 3-gram increments throughout the set, while the balance point is adjusted so that the swing weight of each club is consistent to standard Dynamic Gold.