Electrons have an electric field. By the influence of an external electric field, the electron is attracted or repelled. Electrons also possess a magnetic dipole moment. This moment is coupled to the rotation of the electron (the magnetic dipole moments of an electron and a positron are opposed with the same direction of rotation). When electrons moving through a magnetic field the magnetic dipoles and therefore also the axes of rotation of the electrons are aligned. Now arises a gyroscopic effect and the electron goes under emission of a photon out of alignment again. Detailed see herehere.
The acceleration of electrons can be done not only by electric fields. Under the influence of electromagnetic fields, electrons are also accelerated. With a positive acceleration the energy of photons is transmitted parcial to the electron. In case of negative acceleration more energy is emitted as received. As described above, a magnetic field through which electron moves - not parallel to the magnetic field lines! - influential on the electron too. The electron emit photons and its kinetic energy get lost.