Complex sentence: Greaves usually contained a metal outside with a felt padding inside, the felt padding was particularly important because without it, any blow would transfer directly from the metal plating into the shin, rendering the piece of armour almost useless.
Split into simpler sentences: Greaves usually contained a metal outside with a felt padding inside. The felt padding was particularly important because, without it, any blow would transfer directly from the metal plating to the shin, rendering the piece of armour almost useless.