or, right-click the flow stack frame, then click Step Into Subflow.
The subflow opens in the Message Flow editor and displaces the parent message flow. Message flow processing continues until the next breakpoint that is set in the logical processing of the current message. If there is no further enabled breakpoint at which the flow instance can pause, processing runs to completion and the flow instance is removed from the Debug view.
Next:
If you have finished debugging, you can remove the breakpoints, or end the debugging session: