How/where is the mi watchpoint-trigger / EXEC_ASYNC_WATCHPOINT_TRIGGER raised?

I've found the definition in mi/mi-common.h and the translation in 
mi/mi-common.c, but I have no clue:

1 Understanding GDB:
Where is it actually raised? (I _assume_ it somewhere happens if a 
watchpoint's stop method returns True?

2 Extending GDB:
Can I manually trigger the event from a breakpoint? The reasoning behind 
this is that my watchpoint implementation always returns False after 
setting a breakpoint at the place where the user "expects" to see the 
change (when returning to user code, not multiple times in the library 
code). While it "looks" nice on the GDB console MI-Frontends don't get 
any watchpoint events that way.

Thanks
Simon