🚀 go-pugleaf

What does the following line mean?

    Wait(1 << Window->UserPort->mp_SigBit)

I know it causes my program to wait for a response in the IDCMP port,
but how exactly does this work?

->  means structure member notation for pointers

<<  means shift left, a bitwise operator, which can also be enterpreted
    as multiplication by 2

So, why should shifting the mp_SigBit member of the window's UserPort
one bit position to the left magically cause my application to wake up 
from a sleeping condition and recognize the presence of an incoming message?

                                  -Paul Castonguay