Rabbi Shimon tells Rabbi Yehuda that "Preserve my soul" means "preserve the Hei," for the soul holds onto the Hei. If the soul is deserving when it leaves this world it is welcomed to the World to Come, but if it is not deserving, the angels of destruction push it outside. When David prayed for the preservation of his soul, he called himself pious because he was receiving the flow of Chassadim, and Rabbi Yitzchak says that Chassadim is bestowed by the Righteous, Yesod.
The letter Hei appears twice in the Holy Name of God, the Tetragrammaton יהוה. The first Hei corresponds to the Sfirah of Binah. Binah is vastly important. It is a boundless bank of Light from which we draw the beneficence of the Creator. The Light of Binah also denotes the World to Come - namely, paradise in the here and now, when we are fortunate enough to connect to it.
The second Hei corresponds to our mundane, material world of Malchut. Through his spiritual might, King David likewise embodies the perfection of Malchut. Our soul is preserved and made worthy of the World to Come - the Light of Binah. As Binah's Light is now cast upon the world, the tables are suddenly turned: Binah's rays are directed against the angels of destruction, and they are pushed outside, cast out forever from this world and the next. We are now free to inherit a land of peace, prosperity, and paradise.