I believe because it runs in a subprocess the `PATH` isn't permanently altered.

```sh
$ nvm version

   v0.10.5

$ echo $PATH
/Users/koenpunt/.nvm/v0.10.5/bin:/usr/local/bin:.....

$ NODE_VERSION=v0.11.9 /private/shares/projects/nvm/nvm-exec node -v
Now using node v0.11.9
v0.11.9

$ nvm version

   v0.10.5

$ echo $PATH
/Users/koenpunt/.nvm/v0.10.5/bin:/usr/local/bin:.....