Twisted symmetries, widely studied in the last decade, proved to be as
effective as standard ones in the analysis and reduction of nonlinear
equations. We explain this effectiveness in terms of a Lie-Frobenius reduction;
this requires to focus not just on the prolonged (symmetry) vector fields but
on the distributions spanned by these and on systems of vector fields in
involution in Frobenius sense, not necessarily spanning a Lie algebra.Comment: 27 pages; to appear in J. Phys.