This is the official manual for the latest Org-mode release.
By default Org expands code blocks during tangling. The
header argument turns off such expansions. Note that one side-effect of
org-babel-expand-src-block also assigns values to
:var (see var) variables. Expansions also replace “noweb”
references with their targets (see Noweb reference syntax). Some of
these expansions may cause premature assignment, hence this option. This
option makes a difference only for tangling. It has no effect when exporting
since code blocks for execution have to be expanded anyway.