In what circumstances will a police officer opine a conversation is a drug transaction?

People v. Hernandez (1977) 70 Cal.App.3d 271, 138 Cal.Rptr. 675 is inapposite. There, a police officer opined defendant's conversation with four persons was a drug transaction because they continually looked at defendant's hands during the conversation, and further testified that the shaking of his head from side to side when approached by another group indicated defendant had no more drugs. The court correctly explained, "The officer was no more expert than the jurors concerning the significance of the fact that the four persons kept looking at the area where defendant had his hands. Nor did the officer's expertise add any probative value to defendant's shaking of his head from side to side when he was approached by two other persons." (Id. at p. 281, 138 Cal.Rptr. 675.) Here, the defendants' interest in who crossed out their graffiti and the violent reaction engendered by the shouting of geographical names presented a much more arcane situation, properly interpreted for the jury.