It's a good question, as it didn't make sense to me either. This diagram attempts to explain it:


The Datsun Competition setup is for a dedicated track car:


Competition Anti-Tramp Setup


The tradition street/track setup is like this:


Street/Dragstrip Setup
This is like the other photos, except the front is not solidly mounted to the chassis. It doesn't interfere with normal suspension unless wind-up occurs. Plus it has a little "play" so that it bites down to help traction.