[tongzilla]

So why is dragging a wet mop through impact helpful?

Because you can try to 'throw it away' with your hands and overaccelerate, and still get that sense of heaviness through Pressure Point 3. You can the right feeling even if you do things incorrectly. So you know what it should feel like when you've got a real club in your hands.
Now you've got something to compare against, a feel that you're striving for. So when you do 'get it', you can say to yourself, 'this feels similar to dragging a wet mop. Now I know what this Clubhead Lag stuff is really about'.