Actually, its three different modes.
1. Resistor drop, like SF L1. The power dropped in the resistor is wasted as heat. The LED works around its maximun efficiency.
2. Pulse Width Modulation (PWM). The LED is switched ON-OFF very rapidly. There is no waste of energy in the regulator. The LED works at low efficiency, since -when it is ON - the current flowing through it is close to the maximum allowed.
PWM has been an improper design porting from incandescent world. In traditional bulbs, PWM was invariant to the user since the thermal inertia of the filament; in LEDs, it produces strobing light and force the LED to work in the less efficient area of the I/O curve. It allows for low component count, simplified design, and possibility of using bootstrapped regulators (See the Olight M30 and M20S).
3. Current regulated. Here the regulators sends a regulated current to the LED. In good designs, there is low power loss in the regulator, and the LED is always optimised for efficiency.
Since switching regulators (boost, buck or boost-buck) are mostly used for this design, the output of the switcher must be rectified. It can be done with a Schotty diode (silicon-metal with low Vf) and a capacitor (Fenix, Eagletac), where you can see a slow transition from a level to another.
A better implementation is realised with the use of a synchronous rectifier (SF, HDS) where the transition between brightness levels is immediate - and the efficiency of the circuit is the highest obtainable.
Hope this helps
Anthony
