For a very thorough explanation of Farnsworth Timing check out A Standard for Morse Timing Using the Farnsworth Technique (pdf) by Jon Bloom, courtesy of The Americal Radio Relay League.
The compact version follows: The Farnsworth Timing Method of learning morse code introduced the concept of listening to letters at a fairly high speed, whilst utilizing larger inter-character spacing (silence) to reduce the effective words-per-minute. This enabled the student to learn the sounds of characters as the sound at higher speeds, whilst still making progress at much lower speeds. As progress is made, the effective speed is slowly increased to provide a greater challenge to the student, whilst retaining the same sounds of the characters. Eventually you can see that the effective words-per-minute will be equal to the words-per-minute which is the standard timing for Morse Code.
To highlight the effect here are 12 words at 25 wpm but an effective wpm of just 8 wpm. This is achieved with the effective_wpm command.
require "cw" # farnsworth_timing.rb cw do shuffle effective_wpm 8 wpm 25 word_count 12 ## test_letters end
To save on the typing the command effective_wpm has an alias called ewpm.
require "cw" # farnsworth_timing_2.rb cw do shuffle effective_wpm 12 wpm 18 ewpm 12 word_size 4 word_count 12 print_letters end