Here's the foundation.
3.7.3. Blocking a Weapon strike by laying a Weapon against a Target Area and/or Shield is illegal.
This is typically called "anvilling" in my experience. The idea behind it is that if you violate 3.7.3, you take the appropriate damage to the target area as though your weapon wasn't laid against it.
3.1.5. Hand(s) - Area below the wrist (exclusive). An empty Hand is a legal Target Area. Any Injury to the Hand is considered Injury to the Arm. A Hand on a Weapon or Shield is considered part of that Weapon or Shield.
Now we add in 3.1.5. If I put my hand (hand on weapon) on a target area and my hand is struck... Do I take the damage?