When you hit F5 or choose Debug with a cab selected StackHash will extract the contents of the cab, run one of the debuggers configured on this tab and load the dump file.
On a 64-bit system StackHash automatically detects the architecture of the target crash dump and runs the appropriate version of WinDbg.
You can choose which debugger to use by right-clicking a cab and picking a debugger from the Debug Using menu. From the same context menu you can also extract a cab to the directory of your choice if you want to inspect the contents or keep a local copy to work on when disconnected from the StackHash service.