In our custom implementation, the basic work of a formhandler is to validatate data that came from a form. In our formhandler, we use some manager class and from manager class we call some tool class. In our tool class we will use some repository(In a repository we give definitionFiles=......../xyz.xml). So what operation we are going to perform via form like add, update, delete, show will affect tables in xyz.xml file.
In OOTB supost we are using ProfileFormHandler so it will affect ProfileAdapterRepository, which has xml file is useProfile.xml.
You can enable loggingDebug on the repository component tied to your formhandler to figure out what CRUD operation is happening because of formhandler call.
As per RMishra example if you are dealing with ProfileFormHandler you have to know that it deals with /atg/userprofiling/ProfileAdapterRepository so enable loggingDebug on /atg/userprofiling/ProfileAdapterRepository