I often find musicians confused by the term tremolo and vibrato. Vibrato is the rapid slight alteration of pitch and tremolo is the rapid repitition of a single note. String pull usually happens when the pickup has a strong magnetic field. As the vibrato arm is moved in such a way to loosen the strings, the magnetic field is actually pulling on the string that is made up of ferrous material (reacts to magnetic fields). The plain and inner core of wound strings are made from spring steel with is attracted to magnetic fields. Magnets are used in pickups to magnetize the string. When the string is plucked, it generates an alternating current which is then amplified many times. Some vibratos can loosen the strings beyond any tension. When they are in that position, the strings will naturally be attracted to the pickups. This will cause all kinds of pops, clunks and harmonic overtones. If used and controlled, it can produce some pretty amazing sounds.