Originally Posted by 12XU

I think you may be referencing the Sigma line of headlights which use a ratcheting clamp:



Now, this light is used strictly to be seen and not really to see with. In a pitch black commute, I'd suggest a generator light (would require a specific hub and light combination) or something that uses a battery pack (Marwi makes a decent self-contained one for those looking to save weight).