Theory of Mosfet P channel
After turn on supply on source is 10V, but beetwen source and gate is -10V (gate is invers polarity for source). Transistor go into conduction state but with increase current the voltage on source terminal decrease. Potencial beetwen source and gate decrease and transistor will stop conduction (or better word, current flow decrease). Finaly transistor reach ballance and on source temrinal you get ~6V. 
Because gate is always zero polarization of source have matter.