[glang]

perhaps the diagram from the Suzuki manual will help. The output voltage from alternator changes with revs (the manual says 60vAC at 5000rpm is correct with it unplugged) this is changed when its rectified but its still too high to use in the bikes electrical system.
The three transistors in the diagram to the right of the bridge rectifier are controlled by the regulator to open sufficiently to allow some current to flow back to the alternator. This drops the output voltage of the alternator as it can only produce so much power (=volts x amps) and now more current is flowing. The transistors present a resistance and as Watts=amps x amps x resistance theres an increase in the power to dissipate as heat.