I usually find this on animated pose sets. For example: stimuli's hopping aiko.

How do you use break spline effectively? What are the factors needed to put in mind before you use it?

 I don't know about Poser specifically, but normally breaking splines allows one to shape the intensity of input and output curves of a keyframe independently. For instance your input curve could be smooth and your output linear such that the motion could ease into a key, then abruptly change to constant acceleration.

You've read the book.
Now see the film.
http://www.philc.net/animation1.htm

Next week I'll be selling T shirts :)

Basically, when you have a motion that would be "sinuous" or
smooth in real life, you can set a couple of keyframes and let
spline take care of the rest.   Put a spline break at the start and
end of this motion so Poser won't carry the wave on into other motions.

Say your character is going to nod his head several times.... you'd
set the head's Bend to -10 at the high points of the nod and +10 at the low points.
Saves you the effort of marking every frame in this smooth motion.
When he's done nodding, put a spline break.

Nooooow I know. Thanks guys. Phil you rather sell donuts. They sell faster lol! XD

Break spline is useful when there is an abrupt change of direction. If you have one of stims sets have a look at where the keyframes are and where break spline , or one of the other types is used.