What is usually meant by assumption of risk is more precisely termed primary assumption of risk. It occurs when the plaintiff has either expressly or impliedly relieved the defendant of the duty to mitigate or relieve the risk causing the injury from which the cause of action arises. It operates as a complete bar to liability on the theory that upon assumption of the risk, there is no longer a duty of care running from the defendant to the plaintiff; without a duty owed by the defendant, there can be no negligence on his part. However, primary assumption of risk is not a blanket exemption from liability for the operators of a dangerous activity. The specific risk causing the injury must have been known to, and appreciated by, the plaintiff in order for primary assumption of risk to apply. Also, assumption of risk does not absolve a defendant of liability for reckless conduct.
This defense is commonly used in cases of injuries occurring during risky recreational activities, such as skiing, paragliding, and scuba diving.
Secondary assumption of risk is a rather different doctrine akin in some respects to comparative negligence. The difference was explained by the Supreme Court of California as follows:
In cases involving ‘primary assumption of risk’—where, by virtue of the nature of the activity and the parties' relationship to the activity, the defendant owes no legal duty to protect the plaintiff from the particular risk of harm that caused the injury—the doctrine continues to operate as a complete bar to the plaintiff's recovery. In cases involving ‘secondary assumption of risk’—where the defendant does owe a duty of care to the plaintiff, but the plaintiff proceeds to encounter a known risk imposed by the defendant's breach of duty—the doctrine is merged into the comparative fault scheme, and the trier of fact, in apportioning the loss resulting from the injury, may consider the relative responsibility of the parties.