RE: Torquing head bolts

[Martin Scarr <martins@efn.org>]

Maybe a bit off topic here, but I ran into something that's puzzled me.  I
helped my son-in-law do head gaskets on his '90 Nissan Maxima with the 3.0
SOHC motor.  This is an iron block V6 with aluminum heads.  The Haynes
manual says to torque the head bolts in 5 steps:  23 ft-lbs, 42 ft-lbs,
loosen all bolts, 25 ft-lbs, 47 ft-lbs. 

I've never seen this 'loosen all bolts' step; it doesn't make sense to me.
Anyone have an explanation?  

Thanks
Martin Scarr