[XBL] clarification: explicit children must be redistributed?

Can you please add a little more detail to clarify how point 6 in 
Section 3.5 works. It currently reads:

If the new binding changes which shadow trees contribute to the final 
flattened tree then the explicit children must be redistributed.

I've read section "processing content  elements", but I am still a bit 
unclear about how this step works.