  The current through a helical edge state of a quantum-spin-Hall insulator may
be fully transmitted through a magnetically gapped region due to a combination
of spin-transfer torque and spin pumping [Meng {\em et al.}, Phys. Rev. B {\bf
90}, 205403 (2014)]. Using a scattering approach, we here argue that in such a
system the current is effectively carried by electrons with energies below the
magnet-induced gap and well below the Fermi energy. This has striking
consequences, such as the absence of shot noise, an exponential suppression of
thermal noise, and an obstruction of thermal transport. For two helical edges
covered by the same quantum magnet, the device can act as robust noiseless
current splitter.
