Killer whale attacking a Steller's sea lion
There are many shots that require you to react extremely quickly in order to capture what you want such as these whales I photographed during one of my Alaska workshops. One of the most successful techniques you can use is what I call the "test shot."
The test shot has two primary objectives, exposure and rough focus. When taking a test shot your actual subject is usually not in the frame. For example, these whales break the surface in less than a second and if I wait until the subject is in the frame to find exposure I will be less than successful most of the time. If instead I anticipate where the subject will appear or what the event will look like, I can figure out exposure ahead of time very easily. I can also pre-focus so that the precision focus process is much faster once my subject is in frame and my auto focus sensors are properly placed.
A humpback whale going for it in front of Devil's Thumb
When you make a test shot, it is often times not adequate to set exposure based on the environment/background. You need to anticipate what will drive the exposure. For example, in all three whale shots the critical elements are the white water and the white fins/body. If I make a test shot of just the water and set exposure without considering the extreme tonal difference between water and white water/whale, I will, most likely, blow the highlights in the shot. When I made the test shots for these images, I set my exposure based on white not on the water/background. You can use rules of thumb to underexpose for the predicted highlights but I prefer actual meter data, when I can get it. In these examples, I changed my camera's metering mode to Spot and then I set exposure off of something white. I actually used the boat's wake but a white sheet of paper would work as well. As long as the white is under the same lighting conditions as my subject, my exposure will work perfectly.
This process is simpler when shooting in Manual exposure mode but can be used with Aperture or Shutter priority and auto exposure lock. Once I have my exposure, I pre-focus to the expected subject distance and take a test shot and evaluate. With my exposure and pre-focus set, I am much better prepared to quickly execute the actual shot when the action occurs.
I use test shots all the time to give myself the advantage when something might happen and my reactions have to be perfect. When I am out photographing, people find it odd that I am shooting every now and again at what appears to be nothing. But my "strange" behavior pays off. If you stack your deck ahead of time you will enjoy success in a more consistent and repeatable manner.