A dephlegmator is essentially an inline condenser built into the top of a distillation column. The dephlegmator can be as simple as a copper coil contained within the column. The coil would be hooked up to a cool water supply and water would be circulated through it (in a regulated fashion) to achieve the desired level of reflux. Dephlegmators can be as complicated as several pipes running through a water cooled jacket. Again, the flow of water would be regulated until the desired level of reflux is achieved.
Dephlegmators are sometimes incorporated into column still designs and are devices that work in a similar fashion to the copper packing and plates listed in the column still section above. However, there is one big difference. In the examples above, reflux is the result of passive or "natural cooling" caused by material temperature differentials between the boiler and the top of the column. Dephlegmators use "active cooling" to cause condensation of alcohol vapor and induce reflux action.