The benzyl halide 1 reacts with hexamine to a quaternary ammonium salt 3, each time just alkylating one nitrogen atom. Then the benzylammonium undergoes an acid-catalyzed hydrolysis process.
Depending on the hydrolysis conditions, the hexamine unit might instead break apart, leaving a benzyl amine (the Delépine reaction).
The reaction can also be applied to the oxidation of benzylic amines.  In this way, m-xylylenediamine can be converted to  isophthalaldehyde.