Slanker: Good question.
That bottom rod is your CS. The penis also has two top rods, or chambers, called the CCs. Together, there are three chambers that make up the "tripod" feel. Most men don't recognize the "tripod" until they actually spend some time looking at the bottom part of their penis, such as when they PE. I didn't recognize it until after I started looking at the penis anatomy.
See this article for more of a visual of the CS/CCs:
The Penis Enlargement Gym - Penis Anatomy 101
Hope this helps,
rem