The magnetic field constrains electrons released from the negative plates into tight helical trajectories in the positive tubes. When they strike a neutral gas atom or molecule, ionization occurs.
Since the ion is formed in a highly positive region (with respect to the end plates), the ion's potential energy converts to kinetic energy as it accelerates toward the plate. On impact, it has sufficient energy to sputter titanium which coats all unshielded surfaces (the tubes and cathode plates) with a thin film.
Several pumping mechanisms may then occur: chemical reaction between active gas molecules and the fresh titanium surface; burial of the original ion in the bulk titanium plate; or burial of gas atoms/molecules resident on the surface as the film started to form. The last two mechanisms account for the pump's ability to "pump" inert gases.