By making a very dilute solution of the protein, single molecule FRET lets us look at one individual protein at a time (a very short period of time) as that protein enters and exits the focal point of an excitation laser.  So why do we use a laser?

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Well, first we attach fluorescent dye molecules to our antibody on locations where we want to study movement.  The first dye we used is called  Alexa Flour 488, which we refer to as the "donor" dye.  When it encounters the incoming photons from the laser beam, it absorbs their energy.  The other dye is Alexa Flour 594, or the "acceptor" dye.  After 488 absorbs the energy of the photons, it can transfer this energy to 594.  Next, the excited 594 dye relaxes, emitting this energy once again as photons.  However, these photons are a different wavelength than the ones we started with, as some of their energy has been lost in this process.  When the donor and acceptor dyes are near each other, energy transfer efficiency is high.  Most of the energy absorbed by 488 is transferred to 594.  When they are far apart, however, energy transfer efficiency is low.

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Thus, the intensity of light coming off the acceptor dye tells us about the distance between the 2 dye labels, and gives us a "measurement" of the protein while it moves freely in solution. We use detectors that are sensitive enough to capture this tiny firework, and each one after it, as the process repeats itself for the duration of the protein's journey across the focal point of the laser.