This feature encapsulates the ability to subsitute modified code in a running application through the debugger APIs. For example, one can recompile a single class and replace the old instance with the new instance. This minimizes time spent by the developer to make small, incremental changes to their application.
This feature is required by some tools vendors before they move to JPDA. They
have created hooks in their specialized VM and specialized debugger interfaces
for this functionality.