Simply put, IMHO-it is a new birth, a birth of the spirit. When there is real change in us, we know we do have His Spirit. If there is no change, we are still fully carnal.
Not that I have anything to back up the following but, it is what I have thought for many years. When we are born of "water", I say it is from the mothers womb (first birth) we are in her amniotic fluid popularly called "water" (a women's water breaks when about to give birth...the first birth) The second birth is to abide in Christ the (De-Vine) lol...and we's da branches!
My belief is it all starts with surrender, killing the flesh...renewing our minds...WE renew them, not Christ...His Words will renew our minds and open up some awesome personal stuff!
JMHO