I think that Lashing is very closely related to Steelpushing and Ironpulling. Brandon has stated that similar powers in different magic systems work the same way, but powered by a different Shard. So then Pulling should be like Reverse Lashing a piece of metal to yourself, not Lashing yourself to the metal. That way normal gravity would still affect you.
By that same postulate, filling an ironmind would also be similar to decreasing your weight with Lashing. Exactly as Zas says, manipulating gravity with Surgebinding decreases your spiritual connection with the planet. Filling an ironmind would be storing that connection. So when you're storing weight, your spiritual connection with the planet is weaker, because some of it is going into the metalmind. Then, when you tap, that connection increases. That make sense?
PS: Sorry I don't know all the types of Lashing, since I haven't read more of WoK than the sample chapters on Tor's website. But I understand how they work. Edit: I read the Ars Arcanum, and slightly modified my theories to make sense.
On another note, I've got another question: What would happen if a normal (non-Returned) person drew Nightblood fully and ran out of Breaths? I assume he would still be powerful, but not as powerful as when he's feeding on a wealth of Breaths.