"Engrave on it with signet-ring [type] engraving: "Holy to Hashem" פִּתּוּחֵי חֹתָם קֹדֶשׁ לַי־הֹוָ־ה
In Maseches Ta'anis (2a) R' Yochanan states that there are three maftechos, three keys, in Hashem's possession that He never entrusts to others:
*the key to childbirth
*the key to rain
*and the key to resuscitating the dead.
The Gemara derived this from three verses:
*Hashem does not entrust the key to childbirth to a messenger, as the verse states: "G-d remembered Rachel; G-d hearkened to her and He opened (vayiftach) her womb" [Bereishis 30:22]
*Hashem does not entrust the key of rain to a messenger, as the verse states: "Hashem shall open for you His storehouse of goodness, the heavens, to provide rain for your Land in its time" [Devarim 28:12]
*And Hashem does not entrust the key of resuscitating the dead to a messenger, as the verse states: "Then you will know that I am Hashem, when I open your graves" [Yechezkel 37:13]
These three keys, remarked the Vilna Gaon, are alluded to in the verse: "Pituchei chosam kodesh laHashem" - פִּתּוּחֵי חֹתָם קֹדֶשׁ לַי־הֹוָ־ה -
The acronym of the word חֹתָם [Ches, taf, mem] which means "seal" hints to the following words:
Ches - Chayah [a woman who has recently given birth]
Taf - Techiyas HaMeisim [the resuscitation of the dead]
Mem - Matar [rain]
The verse can therefore be read as follows: "The keys of [pituchei] childbirth, resuscitation of the dead, and rain - ChoTaM - are designated for Hashem's use only ["kodesh laHashem"].
Source: Rabbi Yisrael Bronstein