power supply independent biasing
This current supply is independent of the Voltage because the voltage source isn't the reference for the resistance.

It's a bit easier if you walk through how this stabilizes itself. When Vdd is applied a reference current is forced on M1 because it is diode tied. This reference current gets multiplied by ~K in M2 and that current flows into the resistor which produces a voltage. That Voltage takes away the amount of voltage available for the PMOS M3 mirror. That in turn forces the PMOS M4 to reduce the reference current to M1, so M2 produces less current  which lowers the Voltage drop across R increasing the amount of voltage for M3 and this keeps going until it stabilizes.

As you can see in this example. I never once needed to know what Vdd was. So long as Vdd was low enough that it doesn't harm the mosfets, then the mosfets regulate themselves using the voltage feedback from a known resistor. That's why this is "supply independent (within reason)."