The Petzl Flex is a center bar suitable for all crampons of the French mountain sports specialist. You get a more flexible connection between the front and rear part than with standard bars. Due to the flexibility you can walk more naturally which is a lot more comfortable. In addition to that the center bar will last even longer when you use trekking boots with a flexible sole or large shoes (shoe sizes above 42). The explanation for that is simple: if you use trekking boots with a flexible sole with stiff center bars the force is transmitted to the center bar with every move your feet make. It might become deformed or even break in the worst case. When youre wearing large shoes this effect is even stronger due to the greater leverage.
For alpinists who are placing value on the weight: the Petzl Flex is about 40 g lighter than a stiff standard bar.