cherrybim – there is no chiddush in wearniog tzitzis out, it’s just that in certain communities, like the yekies for instance, the yidden were forced to rely on certain heterim and kulos altz sakana because of anti-semitism, this is why they didnt wear yarmulkahs when they were working, and it would explain wearing tzitzis inside too.
The only actual mekor for wearing one’s tzitzis inside, as oposed to the seforim which talk about wearing them outside as a basic idea and pshat in oresem(both the idea of wearing them when it is dayime and seeing them are pshatim in tha posuk), is the arizal, who ive heard said that al pi kaballah one should wear it inside – however, most people today who wear them inside do not know this, nor do they know kaballah..the use it as an excuse to not look too different from goyim