Hi guys

I just attached a rack to my bicycle, and I'm wondering - Whats the purpose of that springy bit that can be pulled up perpendicularly to the bike?the guy at the LBS said its for clamping stuff like grocery boxes, but isnt the spring tension a bit strong (I tried and it squished one side of the box)...so I was wondering -How do i use it properly?

Regards
TanSC