The namespace upvar command creates a link (just like upvar) from each of the named otherVars in namespace to the myVars in the current context (i.e., stack frame or namespace). All the otherVars must be unqualified.
Note that these are equivalent:
set ns ::foo upvar #0 ${ns}::bar boo
and
set ns ::foo namespace upvar $ns bar boo
However, the second form is more efficient, as requires less string manipulation and parsing.