Question about metalmind medallions

[HeyLookItz he/him]

How do the metalmind medallions that allow you to make yourself lighter work? I understand that they work similarly to the unkeyed gold metalmind that Wayne finds, but instead they store Investiture, which when tapped gives the user the Feruchemical ability to store weight in iron. However, to access the stores of Investiture in the nicrosil, the user should have to already be a Soulbearer Ferring or full Feruchemist. If this is the case, how do characters without this Feruchemical ability, such as Marasi, use the medallions?

[Voidus]

Brandon hasn't completely revealed how that works last I'd checked, but here's his explanation:
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Questioner

So nicrosil.  Wax couldn’t use a blank gold metalmind because he’s not a gold ferring, why can he use a blank nicrosil metalmind?

Brandon Sanderson

So this will all come out eventually but the idea is there are certain ways to connect yourself to magic, to hack the magic and make it think you have the Spiritual DNA that you don’t actually have.  And this is one of the ways.

Questioner

So then the people who made this medallion have this thing that a regular nicrosil Ferring couldn’t--

Brandon Sanderson

Yeah, you’re picking up on it. We’ll dig deeper into it as the series progresses.

[HeyLookItz he/him]

Ok, thanks for clearing that up.

[Slimy_Slider]

I'm going to assume that it's something to do with the fact that everyone has some level of latent Allomantic/Feruchemical abilities from the parts of Ruin and Preservation inside of them, this was why the Mists in Era 1 were able to affect so many people who didn't snap normally, it woke and boosted their latent potential. They weren't as strong as normal mistings, but they had the ability to use the power. If we assume that Feruchemy works in a similar way, then the nicrosil metalmind is just boosting the almost negligible level of inherent power to a usable level.

Just my own personal idea of how it would work.

[Quantus he/him]

3 hours ago, Slimy_Slider said:

I'm going to assume that it's something to do with the fact that everyone has some level of latent Allomantic/Feruchemical abilities from the parts of Ruin and Preservation inside of them, this was why the Mists in Era 1 were able to affect so many people who didn't snap normally, it woke and boosted their latent potential. They weren't as strong as normal mistings, but they had the ability to use the power. If we assume that Feruchemy works in a similar way, then the nicrosil metalmind is just boosting the almost negligible level of inherent power to a usable level.

Just my own personal idea of how it would work.

I dont think that is quite true (at least for Alomancy, we dont know where feruchemy comes from), the whole function of Lerasium is to permanently modify the Spiritweb.  I think Leras was using the Mists (and his more general connection to all Scadrians) to make those manually for each person, not unlike how he could consciously fuel allomancy direcly via the mists rather than burning metals