When you call external code from VA Smalltalk, all of the VA Smalltalk processing and the external function's processing are executed on a single thread. This means that while the function is being run, processing for the rest of VA Smalltalk is halted, including processing for the normal operation of the VA Smalltalk user interface.

To overcome this problem, VA Smalltalk enables you to reroute the execution of any external function onto a separate thread so that normal processing can continue while the function is running.