What is the hazard turn signal?
There are a few different hand signals to use depending on what the situation is.

In general, making a fist behind your back or with your elbow out and bent downward signals to other riders that you are planning on making a stop, potentially a very sudden stop for unexpected reasons; When closing in on road hazards, pointing them out is extremely helpful to the riders behind you, who in all likelihood can't see the hazard directly in front of you and could be caught off guard by it.

When I see glass (/sharp stuff), I usually relay it to the group by pointing my hand at the ground and wiggling my fingers, indicating something sharp is scattered on the ground.

http://www.active.com/cycling/articles/8-cycling-hand-signals-for-your-next-group-ride

http://cyclingtips.com.au/2009/09/verbal-and-non-verbal-communication/