What is the corpus delicti rule in law and what is the effect of extrajudicial statements?

Under the corpus delicti rule, "every conviction must be supported by some proof of the corpus delicti aside from or in addition to [defendant's extrajudicial] statements, and . . . the jury must be so instructed." (People v. Alvarez (2002) 27 Cal.4th 1161, 1165, italics in original.) The purpose of this rule is "to ensure that one will not be falsely convicted, by his or her untested words alone, of a crime that never happened. [Citations.]" (Id. at p. 1169.) "There is no requirement of independent evidence 'of every physical act constituting an element of an offense,' so long as there is some slight or prima facie showing of injury, loss, or harm by a criminal agency. [Citation.] In every case, once the necessary quantum of independent evidence is present, the defendant's extrajudicial statements may then be considered for their full value to strengthen the case on all issues. [Citations.]" (Id. at p. 1171.)