(They clip on to your regular prescription glasses.)
As noted in this Mi Yodeya answer here:
R. Neuwirth in Shemirat Shabbat Kehilchata (16:24) writes that one should not wear ordinary sunglasses in a place where there is no Eruv because they are not considered clothing. However, if the sunglasses are never taken off even indoors (eg. for medical reasons) then they are considered clothing and can be worn even outdoors with no Eruv.
My question here is for the many people I see who do in fact wear regular sunglasses. Are these flip up attachments any worse? Is there more of a Shema Yishlof (fear they will be removed in a Reshus Harabim) concerning them?