Polyfuses?

i have been looking @ understanding how polyfuses work.

i have old atx power supply wan turn bench supply, want limit available output 3a on available voltage rails support current.  looked @ dangerous prototype's board, outputs rated @ 1.25a.

http://dangerousprototypes.com/docs/atx_breakout_board

therefore looking make own board can fit case , pull of digital volt/amp meters on it, want make sure understand how polyfuses work before go order bunch , find out clueless.

from have read placed in series power source , "max amp" same "max amp" on regular fuse/breaker.  if read "max amp" there "trip current" "above max" current.   so if 3a max amp poly fuse .1a trip current hit 3.2a reason fuse activates.  part understand.

what don't understand how polyfuse's resistance comes play.  polyfuse have been looking at.

http://www.mouser.com/productdetail/bourns/cmf-rl50a-0/?qs=sgaepimzzmsxr%252bbxi4wrumne3blcc5i8ar6gfw0%252bmmy%3d

this has 50 ohms resistance.  how factor circuit?  once current > 3.1a it's factor or part of circuit , > 3.1a current resistance different resistance?

they have conductive powder (ie metal dust) in polymer matrix.  particles
touch each other conducts normally, above current various
resistances @ particle interfaces means start heat noticably.  this
causes polymer expand, pulling particles further apart - current still flows
but resistance increases heating increases , runaway reaction
and expansion pulls particles far enough apart rapidly increase resistance
(the fuse "opens") 

there 2 stable states, enough voltage available remains hot , carrying a
small current (enough keep hot) , has high resistance - current limited,
the voltage across high.

with low current stays cool , low resistance , low voltage drop.

thus automatically resets (slowly) after power removed.

also temperature rise limited hot , no current can flow, there
is limit how hot gets - not going destrroy itself

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