After initrd image unpack and mount to / (rootfs, tmpfs):

mount /sys and /proc
init-top hook
init-premount hook
    local-top hook
    local-premount hook
    mount realroot to /root
    local-bottom hook
init-bottom hook
move mount /sys to /root/sys
move mount /proc to /root/proc
exec run-init(klibc)
    delete rootfs contents
    overmount from /root to /
    chroot and chdir("/")
    exec /sbin/init