http://www.purl.org/tcl/home/man/tcl8.6/TclCmd/my.htm
The my command allows the invocation of any method defined by an object upon itself, including non-exported methods.
It is really a separate command that is created in each object's namespace, so it can be invoked from outside the object if you know the name of the namespace or if you use namespace code inside the object to wrap the invocation. This is useful for callbacks like variable traces or bind event handlers.