  We analyze quantitatively the generation of a continuous beam of atoms by the
periodic injection of individual packets in a guide, followed by their
overlapping. We show that slowing the packets using a moving mirror before
their overlapping enables an optimal gain on the phase space density of the
generated beam. This is interpreted as a Maxwell's demon type strategy as the
experimentalist exploits the information on the position and velocity of the
center of mass of each packet.
