Searches subtasks of standard issues which satisfies the provided subquery. See also: parentsOf()
Note: Since you need to enclose the provided subquery using doble quotes, you must to escape any doble quotes using in the subquery by preceding a backslash after the double quotes, alternatively you can use a single quote in some cases.
Let's see an example...
This will not work
issue IN subtasksOf("resolution = "Won't Do" AND status = Closed")
Searches for all subtasks of closed issues which their resolution have been set to "Won't Do"
This will work perfectly
issue IN subtasksOf("resolution = \"Won't Do\" AND status = Closed")
Searches for all subtasks of closed issues which their resolution have been set to "Won't Do"
issue IN subtasksOf("status = 'In Progress' AND reporter = currentUser()")
Searches for all subtasks of in proress issues reporter by me