What is the "either/or" rule in the context of sexual assault cases?

"Emerging from recent cases dealing with the problems arising when a crime is charged and the evidence describes several acts, any one of which could constitute the crime charged, 'is the so-called "either/or" rule: ... either the prosecution must select the specific act relied upon to prove the charge or the jury must be instructed in the words of CALJIC No. 17.01 or 4.71.5 or their equivalent that it must unanimously agree beyond a reasonable doubt that defendant committed the same specific criminal act. [Citations.]' [Citation.]" (People v. Callan (1985) 174 Cal.App.3d 1101, 1111, 220 Cal.Rptr. 339, fn. omitted.)

While the "either/or" rule as stated above will cure a failure of election by the prosecution in the usual case where evidence of several specific acts is presented, any one of which could constitute the crime charged, the rule can have no application in a case such as the present one where there is a failure to present evidence of any specific act to support the charged crime. Where, as here, the evidence is that many of such acts were committed over an extended period of time, it would be impossible for the prosecution to "select the specific act relied upon to prove the charge" and equally impossible for the jury to "unanimously agree beyond a reasonable doubt that defendant committed the same specific criminal act." As stated in People v. Williams, supra, 133 Cal. 165, 168, 65 P. 323, "Such a trial, upon a charge so indefinite as to circumstance of time or place, or any particular, except by the general designation, would be a judicial farce, if it were not something a great deal worse."