Great question! The APF (Advanced Pulse-Frequency) Transmission System is a new, proprietary feature developed by Fenix and first implemented on the new TK11 TAC flashlight. This is a brand new, industry-leading technology that you won't find anywhere else!
The APF system allows Fenix to push the envelope in a lot of ways.
-Traditional switches are limited by current, making it difficult to achieve brightness above 2200 lumens in a single-battery light. APF makes it much easier to achieve higher brightness levels, which is great now that we're seeing a lot of lights using the more powerful 21700 battery options.
-APF also makes it easier for Fenix to implement multi-functioning tail caps like what is seen in the TK11 TAC.
-APF allows a simpler body design as well. Traditionally, flashlight bodies with multi-function tail caps have needed to have double-layer bodies for proper current conduction. APF allows a single-layer body to achieve more reliable lights.
In short, APF allows tail-switch controlled flashlights to reliably reach outputs above 2000 lumens and makes it easier to implement multiple functions in a tail cap.
It can work with either mechanical or digital (electronic) switches.
We can see the benefits of the APF system with the unique multi-setting rotary design on the TK11 TAC.
The APF system does not require HALL effect sensors so it is not affected by magnetic fields. It allows for features like the rotary switch to adjust the settings of the TK11 TAC to choose between Lock, Duty Mode, or Tactical Mode.
The setting of the rotary switch on the tail cap is communicated via APF through the body to the control board in the head. The control board then translates the button presses into output changes based on the setting.