In PowerPoint 2002/XP and above, you can add motion path animation.
To do so, select the line, click Slide Show > Custom Animation.
Click Add effect > Motion Paths.
Select a direction of your choice.
You can then adjust the motion path, the speed, and etc.

If I understand correctly, what you are trying to accomplish is a pretty
easy task.  After making the shape (line) right click on it, and go to custom
animation.  Select the option 'peek' (could be others, but this has proven to
work) and from there, you can select which direction you would like the line
to go to.

This is invaluable information, if you're like myself and you're company
relies on you to put together flow charts, or any type of presentation that
will make use for this nifty trick.

Depends on your version of PowerPoint.  Tell us your version and then maybe
we can help.  :-)

Austin Myers
MS PowerPoint MVP Team

Provider of PFCMedia, PFCPro, PFCExpress
http://www.pfcmedia.com