We avoid the aspect of the project that is at the root of the risk.
In contrast to RiskMitigation, where we have to pay even if a risk doesn't materialize, or a RiskReserve where we tie up resources until a risk expires, RiskAvoidance at first appears free.
But the downside to this approach is that by avoiding the risk we also avoid the possible benefit from taking on a more risky task.
For example, a TimeAndMaterialContract avoids the risk that we will not get paid for the work we have done or the money we have spent because we did not adequately predict the time or cost of the project.