High-Speed Sync (HSS) allows you to use your flash at a shutter speed that goes above 1/200 of a second, which is a normal flash sync speed on most digital cameras. HSS allows you to set the exposure for a dramatic sky and still get a good exposure on the model as well.
For the left picture above, shot in ambient light, Jared set the exposure for the model's face in order for it to be properly lit. However, this will make the background way too bright. Here is how you solve the problem by using HSS:
Step 1. Set your exposure for the background of the shot. For the right picture above, Jared wanted to expose for the dramatic sky (shutter speed 1/500 sec). If you don't use flash, this will make the model very underexposed.
Step 2. Set your Air Remote TTL on high speed sync mode. If you use Profoto A1X, this will be set automatically when you are using a shorter shutter speed.
Step 3. Use an off-camera flash (A1/A1X, B10/B10 Plus, B1/B1X, B2), and position it where you want the light to hit the model. For the image above, Jared used his flash positioned to the right of the model.
Step 4. Attach an OCF Softbox 2x3' (or an umbrella for the A1/A1X) to get a soft light on the model to match the scene.
High-Speed Sync (HSS) is all about being in control of the light. In other words, having HSS as an option allows you to choose your exposure settings based on your artistic intent rather than the weather forecast.
Avoid using HSS to try to freeze motion. Use Freeze mode for best results when you want to avoid motion blur.