This plan show us how to create a new layer of 3v (Let`s call this one of 3vS). Exist two voltages, 15v and 3v, the first is always on, and is on the left side of the scheme, the second is 3v on the mosfet in the top right.
The SPST Switch is simulating a action of PCH (Chipset Intel), this signal actives a lot of circuit like that. The signal comes when we turn on the machine.
What is the point of that new layer?
Well, all resume in Security and organization. If any electronic component burn or short, that issue will not burn the rest of circuit, stopping in the first mosfet. The organization is about separate layer from others, for diagnostic a problem, for example, we can just isolate a suspect layer and test.
Functional
There are 3 Voltmeter, you can see 3 different voltages (or more) in the oscilloscope. The voltmeter of the right side is the most important one.
