Why does pion decay to a muon show parity violation?

[positron]

Consider this decay:
Pion+ -> muon+ + neutrino
It says that one can check the longitudinal polarization of the muon+ and this would confirm that the parity is not conserved. Can you explain what this means in simple terms?

 

[Astronuc]

Where is it stated "longitudinal polarization of the muon+"? Textbook?

Lets refer to -
http://hyperphysics.phy-astr.gsu.edu/hbase/particles/piondec.html
which doesn't answer the question, but is nice background.

This might explain it -
http://tcmms.ca/~jess/musr/cap/pidk.htm

and some interesting background "The Failure of Parity in Weak Interactions" by Nick Cowan
http://www.astro.washington.edu/cowan/school/parity.pdf

And for additional reading -
http://www.lbl.gov/abc/wallchart/chapters/05/2.html

and for
http://pibeta.web.psi.ch/docs/publications/tom_diss/html/web_diss-2.1.1.html