Force = Mass * Velocity

Pressure (another name for force) is rated in pounds per square inch.

In other words to cause damage you need to apply a maximum of force across a minimum of area. This is why a knife point pierces you but a tabletop does not even if you impact that tabletop with substantially more force. Plate armor takes the concentrated force of a sword edge and (if the armor material does not fail) spreads that across the dimension of the entire plate. That force then passes through the plate to the underlayment (thick padding that absorbs impact and sweat, lots of sweat) before impacting the body. IF a sword edge composed of 1 square inch delivers 100 pounds of force, and it impacts a breastplate that is 1 square foot, then the force that is transferred from the plate is only a little over half a pound (no it is not that clean there is substantial force loss that I am ignoring). This is what happens when the armor works and does not deform.

If the armor deforms under the blow then instead of the force of that blow being spread across the entire under surface area, it is transferred using only the surface area of the deformation, minus the amount of force that was consumed to create the deformation. This event is known in scientific nomenclature as ouch, or in the vulgar as "oh crap, that hurt, you said this stuff would protect me, I think I am dying"

The dissipation of the initial impact depends on the strength of the material the armor is made from, and the angles involved with the application of the force. In iron/steel plate armor, a certain flexibility of the metal is needed, but only to keep it from shattering. In most materials hard=brittle, C does not have this problem, it simply does not break or deform regardless of the amount of force.

Because of this property, you can actually do some really fancy things with force redirection by actually covering the outside of the plates (you could actually go without the plate) with layers of thin wires in geometric formations that would actually take the force of the incoming strike and dissipate it (actually send the force off at an angle as a shear) before it could contact the base plate and be transferred to the underlayment and ultimately the wearer of the armor.
``_/\_
_/\__/\_ as a crude example

Randland folks would probably lack the trig/geometry/calculus to actually design such a lattice, not to mention the manufacturing capability to extrude the hair thin wires and precisely fasten them, but it is a neat intellectual exercise. Theoretically, you could actually make a breastplate that could take the incoming force of an attack, and channel it completely around the protected body (think of how the pavement breaks under Iron Man when Hulk pounds on him instead of Tony Stark turning to jello).

The net result of hitting it with a sword would be like hitting a steel wall, more painful for the attacker then the "victim" because the force of the strike would be reabsorbed by the weapon that made it.