How does underdriving a powerful (eg. 50w, 100w) LED chip work?
LEDs are best regulated by controlling the current through them. Controlling the voltage across them can lead to excessive current flow and possibly killing the LED. See this typical V_I characteristic: -



Above is a typical LED (blue) lowish power LED characteristic. Note that with 2.3 volts applied hardly any current flows into the LED. If this voltage is increased to 2.7 volts, 20mA flows. Given the natural variations in the manufacturing process, it's difficult to control LED current based on assumptions about the forward V-I characteristic.

Even if you did get the voltage right, with high-power LEDs and their self-heating the characteristic might shift and what you thought was (say) 100mA rises up or falls back 50mA in a few minutes.

Best practice is to control the current fed to a LED either by a series resistor (low power applications) or a switching current regulator (higher powers).