An indirect process is one which has no causal agent and no identifiable sequence of stages. The outcome of an indirect process results from the simultaneous, collective interaction of all agents of the process. While each agent performs the same behavior, the agent behaviors are independent of one another.
The process of students forming a bottleneck as they hurry through a narrow doorway when the school bell rings is an indirect process. The outcome of the bottleneck is not caused by a single student nor is it sequential; rather it results from the simultaneous, collective action of all of the students.
For more information, see:
Chi, M. T. H. (In press). Three types of conceptual change: Belief revision, mental model transformation, and categorical shift. In S. Vosniadou (Ed.), Handbook of research on conceptual change. Hillsdale, NJ: Erlbaum.