I learned that "anticipation" was essential to four-handed dentistry — as it is to anything complicated and delicate requiring more than one person’s skills. As a dental assistant, I had to know a procedure well enough to be able to anticipate the instrument or materials required before they were needed and to have them ready at precisely the right time. I also had to know when to suction, how to cut and fit those rubber thingies they use to keep stuff from falling into your mouth and to be able to prepare the gelatinous goo used to make moulds.
Needless to say, as an untrained wife of the dentist, I made a number of annoying mistakes. Fortunately we didn’t bark at each other over the head of a patient, whose trust we had to maintain. Nowadays, I don’t think untrained assistants are allowed to help "chairside" unless the dentist is working with the Medical Missionaries in a third world country and there is no choice! I’ve done that, too.