Correct way to pull using subtree merge without guesswork?

Hello,

can someone please tell me, if this is the correct syntax for using "git pull" 
to pull in "subtree merging strategy", if I don't want to allow git to guess 
where to do the changes:

git pull -s subtree -X subtree=path/to/subtree externalproject master

Returns with success, but is "-X subtree=" the right way to force a subtree path 
and disallow git to autodetect the subtree?

$ git --version
git version 1.7.4.4

Thank you very much in advance

Yours

Manuel