When performing certain actions such as repairing or healing, survivors may encounter a skill check. These are quick-time events which require you to press a key (default: space) at a certain time in order to progress through your action. Should you miss these checks, or fail them, your progress through that action is reverted and you will alert the killer.
If you pass a skill check adequately, your progress will continue and you will receive a sum of points. However, should you hit your check when your marker is within a smaller zone, you will hit a "great" skill check, where your progress is boosted slightly and you receive a bonus amount of points.
These checks happen randomly, but will feature an audio cue before they appear. Be aware of this possibility at all times to avoid being surprised.