The feeling at the base of a penis just before ejaculation.
A sure way to know that
a) You've done your job and its about to be over
b) You know you have time to escape the juices of love.
Sarah was blue in the face after giving head for four hours, when at last she felt the fizzling. She knew her work was almost done. She took one last gasp for air and sprinted to the fizzle line.