This is a great new little feature: if a build fails, you can fix the problem and then continue the build without resetting the log.
The screen pic above shows that a build has just failed, and the Restart build from selected action button becomes enabled. At this point you would normally switch over to the design view, fix whatever needs to be fixed, select the action to restart from and then restart the build. This is particularly useful if you have a long build process and you don't have time to start from the beginning again, or maybe you have a sequence of actions at the start of the build that you really don't want to rerun (maybe they're incrementing version numbers or something). Don't confuse this with the “Run from here” function - Run from here starts a new log file, resets all the build status and starts the build from the selected action.