Set all GPIO to a known value

P1OUT = (P1OUT7 << 7) + (P1OUT6 << 6) + (P1OUT5 << 5) + (P1OUT4 << 4) + (P1OUT3 << 3) + (P1OUT2 << 2) + (P1OUT1 << 1) + P1OUT0;

this  code is  used for  setting Gpio to known value , but   I dont understand , for  examplle if 

P1OUT = 10011000

then :

P1OUT0 = 00000000

P1OUT1 = 00000000

P1OUT2 = 00000000

P1OUT3 = 00001000

P1OUT4 = 00010000

P1OUT5 = 00000000

P1OUT6=  00000000

P1OUT7 = 10000000

Can  me  explain how the  initial  expression with  left  shift  operator  is  right?

Thanks.

Franz