To add on to what Ross said:
For MPPT, it's not as simple as just turning it on and off proportionally to the speed. This is simply frequency modulation, and the duty cycle remains (roughly) the same throughout the range of operating speeds. This doesn't really do much good in terms of controlling the power going through the circuit. To control the power and convert it, the *duty cycle* must be modulated, which is the percentage of the time the switch is on vs. off, regardless of frequency.
The RPM of a shaft (in this case the rotor of the alternator) is typically sensed by something such as a hall effect sensor, a kind of solid state magnetic switch (which has no moving parts). More commonly, it is just picked up directly by the AC coming from the coils in the alternator, and translated from there. Either way, the point is, there are no moving parts associated with "reading" the RPM of the shaft.
The output from this is then in turn used by a microprocessor to compare the speed against a pre-programmed table, which the processor then uses to determine the proper PWM percentage to modulate the MOSFET (type of transistor, in the buck converter section) that turns the juice on and off to the inductor (which is where the "extra" energy gets stored in the process of converting it to another form).
This has to happen very quickly in order to be effective; ***MUCH*** quicker than can be done with anything mechanical.
In the DIY versions of a buck converter, the speeds that they can run at are typically restricted by design limitation (imposed by the materials and construction methods available to the average Joe). 20kHz is ok, and is probably typical, although a little too close to the audible range for my taste, but in terms of design, is sufficient to operate reasonably well with commonly available parts. Personally, I like 25-30kHz, just so I don't have to ask myself "what is that WHINING sound?" every so often...
Faster is better, to a point, at which time the switching losses (the time it takes the MOSFET to turn on and off) become excessive, and heat becomes a problem. Heat=Lost power.
There's a lot more to it than that, but this targets the point you're at right now. Hope it helps a little.
Steve