Blow out happens when the ratio of air to water surrounding the prop gets so great that the prop is no longer biting the water, in fact the prop is now trying to drive itself through air (or a relative vacuum).
When blow out occurs, the prop will loose its bite in the water possibly causing a loose steering feeling or a violent right turn. The speed at which blow out can occur will vary depending on boat design, lower unit design, and prop design/pitch. A blow out ring helps to prevent this. Available for 2.0, 2.4 and 2.5 liter motors, (not available for 3.0 liter).