I understand the VSG as a kind of alignment signal. The dip to -1 for 5ms serves as a signal to any Sounds that are aware of that signal that they have 5 ms to quickly fade the amplitude to zero before the actual trigger On. For example, in an ADSR or the AD of a Sample player... It helps to get things nice and click free, so gates don't interrupt abruptly (which is what can cause clicks).