There is a detail here: in general, permutation tests consist in resampling the residuals of a model by permutation (as the name rightly says). Sign swap is a peculiar case (relatively frequent in functional neuroimaging, where on aims at detecting some signals of interest), that is limited to one-sample tests: in that case, there is not much you can shuffle, but you can still create a a shuffled distribution by assuming symmetry about some reference value, hence the sign swap. 
Could you clarify the explanation here ?