It looks to lock around targets in a square outline with 4 white triangles at each corner.
The red highlight is difficult to see sometimes, so this would be helpful.
Found a video showing it:
https://youtube.com/watch?v=mZISlZ2lCzg
Edited by jelliedsoup on June 27, 2014 10:40PM www.youtube.com/watch?feature=youtu.be&v=Ks8_KGHqmO4