I looked at the page and it says Buck current under the "Circuit operation mode" field. Buck regulation is a term for circuitry that relies on having a power source that is greater than the amount needed to turn on an LED. It "bucks" the power down to a level that the LED can safely handle without melting down or over heating. As opposed to one that utilizes a power source that is lower than what the LED needs to turn on, such as a single AA cell that is "Boosted" to produce the 2.5-3 volts that many LEDs need to light. Yes those are referred to as "Boost circuits".
Pulse Width Modulation is more commonly seen in the outputs other than "turbo" or maximum, because it is the easiest way to make it variable within the chip handling the different options. So while you ~may~ see it when selecting medium or low in a buck circuit controlled light, you are MORE likely to see it in a light that is using a boost circuit because the LED already demands more than the power source can supply, so pulsing it to boost it up to the required level is a necessary thing...but some designs make it less visible to nearly imperceptible on the LED side. If you limit the output to one single level, you can make the pulsing virtually undetectable by building in smoothing to the output. This requires more components so...costs more.
As to efficiency...well that just depends on who makes it. Some companies are really good at it while others are terrible and you would be better off connecting your LED to your power source through a resistor only. In fact some companies do exactly that. Heck some don't even have a resistor in line and just rely on the internal resistance of the batteries to prevent the LED from melting.
