The voltage "regulator" does so via the back door. It "senses" the battery voltage and assumes that all the switches and contacts between it and the battery are pristine and new.
It "regulates" the battery/bike voltage by controlling the alternator output strength via the power it gives to the field coil, which, in turn, controls the magnetic field the alternator functions within.
The Vreg has three output states; Balls-to-the-wall, treat things gently, and die-you-bastard-die. (Yes, the Vreg has no couth. Besides, it's old and cantankerous.)
When the battery is low, the vreg puts full battery voltage to the alternator, nailing the balls-to-the-wall, forcing the alternator to make as much power as it can, given the rpm of the crankshaft, wimpy at idle and "burly manly" at higher RPM.
When the vreg notices that the battery is fully charged (14.5V), it removes one of the figurative nails by inserting a 10 ohm resistor between battery and alternator field, reducing power delivered to the field coil. The magnetic field diminishes, and the alternator output power, also diminishes. Wimpy becomes wimpy-whelping-pathetic, and "burly manly" becomes dainty.
If the battery voltage sensed by the vreg EXCEEDS 14.5-15V, the vreg not only removes all the nails instilling a sigh of relief, but curtails life support for the alternator, by removing ALL voltage delivered to the alternator field. In rare cases, it also sends funeral flowers and a Japanese flag to drape over the burial coffin. But, such events are not widely publicized by the **ahem** current media. Anyway, in this state, the alternator is just a useless lump and doesn't even whimper no matter what RPM it spins at.
So, that's the three states the vreg induces for the bike via the two internal contact sets. At rest, its at the Balls-to-the-wall contact, bypassing the 10 ohm resistor. When the battery is charged, neither contact is made, and when the batt voltage is too high, the upper contact is made, shorting the alternator field to ground, and threatening burial.
The three states can't always be observed by mere mortals as the vreg is a rather fickle device. See, when the battery voltage gets too high, and the vreg turns the alternator down, the battery voltage drops, and the Vreg panics and turns the alternator back on, the voltage gets too high again, and the cycle repeats. The contacts can actually chatter, and the frequency changes with the engine rpm/alternator output strength. But, you really shouldn't be looking at these while your are riding. Road hazards, and all that, you know.
Then there is the battle within the Vreg itself, among the two sets of contacts. When the contacts make/break the current flowing through them, there is heating and some mechanical wear. Over time and overwork (and sometimes maladjustment), pitting and burning deposits can interfere with their operation. Pitting can lock the contacts together such that the vreg isn't strong enough to pull them apart, and a carbon and dirt coating can prevent them from making a contact while closed. This makes nail delivery and extraction problematic, with a side effect of placing lot's of question marks, a few exclamation points, as well as characters from the top row found on standard keyboards, above the motorcycle rider's head. These question marks and other undesirable characters can be removed by reading forum posts, and then cleaning and adjusting Vreg contacts as outlined in the Honda Shop Manual.
Cheers,
