In short, what the flip-flop does for you, when measuring frequency, such as in a tachometer application, is it negates the need to measure the duration of the high portion of the period, then switch context and measure the duration of the low portion of the period. The time it takes to switch contexts (jump to another section of code) will be lost, and your measurement will be innacurate.

The flip-flop makes one long pulse, spanning the entire period of the tach signal, so that a single PULSEIN command can accurately measure the duration of the period.

For an explanation of the flip-flop idea go to the following Parallax URL:

http://www.parallax.com/dl/appnt/stamps/bs1appnotes.pdf

Scroll down to the "Practical Pulse Measurements" section.

For the complete blow-by-blow as to how I implemented rotor tachometers on my helicopter go to the following URL:

http://www.basicmicro.com/downloads/docs/RRPM.pdf