How to understand 'make oldconfig'?

Hi,

    I have tried to understand 'make oldconfig' command while
configurating kernel. I do some experiments and get the following
ideas:

1. When there is no a .config in /usr/src/linux,
    (1)  If there is no a /boot/config-x.y.z, make will ask some
questions and then produce a .config.
    (2)  Otherwise, make will copy the /boot/config-x.y.z to
/usr/src/linux/.config.

My question: According to the messages generated by make, I get that
both 'make defconfig' and 'make oldconfig' will gerenate .config based
on 'i386_defconfig'. Why does 'make oldconfig' ask some questions,
while 'make defconfig' does not?

2. When there is a .config in /usr/src/linux indeed, make do nothing
but generate a copy of .config, namely .config.old.

My question: According to Love, "After making changes to your
configuration file, or when using an existing configuration file on a
new kernel tree, you can validate and update the configuration: make
oldconfig." But, whether I edit .config and make some changes or I
copy an existing .config into /usr/src/linux, 'make oldconfig' seems
do nothing. So, how can I understand 'make oldconfig' in this case?