Consent must always be voluntary, without being subjected to threats, fraud, coercion, or deceit. Consent is an affirmative yes.
In order for consent to be enthusiastic and affirmative, all parties involved must communicate and negotiate their individual interests, desires, protocols and limits. With informed consent, all parties have all the information necessary for a safe, healthy and fun scene or dynamic.
One must be an adult, alert, and of sufficient mental capacity to give consent. Each person has the legal obligation to insure that the person with whom they are seeking consent is fit to give it. To give consent one must be clearheaded and not impaired by any substance. Be aware that situaltional endorphins can impair one's judgement as well.
Even an "enthusiatic yes" isn't a blank check to do anything. If there is a desire to move outside of whatever has already been agreed to, then there needs to be more negotiation. Prior consent is not a current yes.
One yes is not a continued yes. Use of safeword (at this event it is RED) means stop immediately because consent has stopped. Ignoring safewords or playing without safewords is not acceptable and are considered consent violations.
Playing without a safeword (or safe sign) is not consent and is not acceptable.
Anytime "serious bodily injury" occurs during a scene, it is possible that criminal prosecution for assault will take place even when consent was clearly and validly given for the activity that caused the injury. Some courts have even found that "minor harm" is an offense. Don't push the limits.