An MLM spillover simply refers to the fact that a member in your upline may decide to place a recruited downline under you which would in turn give you profit as soon as that downline performs.
At the same time, you yourself may choose to place your new downline under a person in your team whom you feel needs it more than anyone else. This would also be an effective downline retention move that you can use because the person under your team with whom you placed your recruit would feel your support and would be motivated more into staying and being active in your group.