The question posed in the title concerns the explanation of a well-practised technique in ECR ion sources for increasing the output of the highest charge states of the ions of interest.For a long time most accepted model was that of ion cooling,being a'single-particle'effect.Two recent papers,likely inspired on earlier work.are proposing a'collective'effect due to non-linear plasma-wave interaction,giving rise to turbulent heating.The mixing gas will in that picture reduce the heating.A few experiments are suggested to help unravel the problem of stating which effect is dominating.