Edit controllers that use an edit control connection often require that the play VCR or camcorder also have a real-time counter. Desired scenes are marked using the player. The player finds and plays those scenes, triggering the recorder to record them. Most synchro-edit systems are limited in the number of scenes they can hold and accuracy can be limited. The advantage of synchro-edit is that it is simple and, if your camcorder and VCR are equipped with the feature, requires no additional equipment.
Pros can even cut single frames together.
True frame accuracy and single frame editing are not practical on ANY home, or even semi-pro, system, regardless of what you may be told, because the cheapest VCR that can record a single frame or achieve absolute frame accuracy costs several thousand dollars. But you can come pretty close to frame accuracy at home. The precision of a home system depends on the camcorder and VCR, as well as on the edit controller. With some equipment, consistent accuracy within 2-3 frames is quite achievable. Most equipment can easily be accurate to within a second or two and with care, you can likely see accuracy to within a few tenths of a second.