When a program is running under the debugger, by default the "run all threads" ("RA") mode is turned on. In this mode, you step through only one thread at a time, but the background threads run normally. If a background thread reaches a breakpoint, it returns control to the debugger and becomes the current thread. The last debugging mode you select is saved into your ".ADB" file, so the default mode applies only when you do not have a ".ADB" file.
At your option, you can execute one thread at a time in the debugger. This allows you to trace a thread without interference from other threads. When a new thread starts, the debugger informs you, but continues tracing the parent thread. You can switch to another thread with the "ST" (Switch Threads) command.
You can find a list of the current threads under the "Run" menu item. This list shows you the current program and address where each thread is executing. You can select the appropriate menu item to switch to that thread as an alternative to the "ST" command.
The debugger can manage only ten threads simultaneously.