The AC Encyclopedia says:
EARLY BLADES
When Altair wielded his Hidden Blade during the 3rd Crusade, he did so in the traditional manner followed by all members of the Brotherhood. The blade sat on a spring-loaded mechanism operated by a ring cuff attached to the little finger. This cuff triggered a switch that allowed an Assassin to quickly extend or retract the blade with a simple flick of the wrist.
RENAISSANCE BLADES
The Hidden Blade was eventually redesigned by Altair...Ezio inherited his father's Hidden Blade and had it repaired and modified by Leonardo da Vinci. These new weapons had been altered for greater stealth, and thus the ring cuff switch had been removed to make them less recognizable. Instead the mechanism was operated by a pressure switch inside the bracer, which could be depressed by flexing the wrist to extend or retract the blade. The loss of a finger was also no longer required.
MODERN BLADES
Desmond Miles also used the Hidden Blade. His first blade was an unmodified weapon similar to Ezio's. Desmond later used a simplified version, which consisted only of the blade, its mechanism and a leather harness. This light blade was as versatile as Ezio's.
There you go.
