As you approach a physical oncoming hazard, take the arm on the side of the hazard behind you and point across your back in the direction the cyclist behind you will need to move in order to avoid it.
In the UK, the hazard usually approaches on the left in the form of a parked car or similar, so the left arm is normally used.
In the event of a late notice Hazard, or one which requires more urgent action, it can be useful to emphasize the hand signal by moving the hand back-and-forth in the direction of the point as shown below, accompanied by a suitable call such as “Heads-up!”
As well as parked cars, this signal is also used to enable the peloton to safely pass other riders, runners, dog-walkers etc. It can sometimes be helpful to give a shout out as to the type of hazard, particularly if it is unusual, for example “Runners”.