My understanding is that a tiny amount of virus capsules are trapped by a moisture barrier on the mask. But that would only have a marginal protective effect for others in the extremely specific circumstance that: 1) You are literally sick with the virus or asymptomatic transfer is a thing (not sure if that was even proven?), 2) You are only next to a person in a non-confined space for a short window of time - shorter than that and there's no risk in the first place, longer than that and enough virus gets through anyway. 3) The mask is fitted properly, you don't have a beard, and it hasn't been subsequently handled by your grubby fingers.
And this is just a theory. I don't know that it's been experimentally demonstrated.
My understanding is that a tiny amount of virus capsules are trapped by a moisture barrier on the mask. But that would only have a marginal protective effect for others in the extremely specific circumstance that: 1) You are literally sick with the virus or asymptomatic transfer is a thing (not sure if that was even proven?), 2) You are only next to a person in a non-confined space for a short window of time - shorter than that and there's no risk in the first place, longer than that and enough virus gets through anyway. 3) The mask is fitted properly, you don't have a beard, and it hasn't been subsequently handled by your grubby fingers.
And this is just a theory. I don't know that it's been experimentally demonstrated.