The darbahai possesses some supreme capacity to thwart any dosham - be it with reference to
(1) lack of austerity or ‘madi’, or
(2) pollution, in spite of the sthala-suddhi done at and around homa –kundam,
(3) negative vibrations and thoughts of any one present there or
(4) the invasion of such energies including harmful radiation from any source.
In all japas, the doer is supposed to hold the darbhai in his hand till the japa is over.
The bunch of darbhai held during the sankalpa is kept in front of the man
in such a way that he, as doer is protected from any debilitating energy.
The darbhai spread on homa kundam serves the purpose.
The darbhai used in pavithram serves the same purpose.
Darbhai is used in all actions that are done
to communicate, transfer, or invoke some results from beings of non-earthly nature.
That is how their relevance comes in tarpanam.
If some one discards darbhai, it means he is without protection
and can not guarantee that his offerings or prayers are being sent to
or received by those to whom they are intended, in their fullest intent or import.
Since we can not know or perceive the kind of obstructions taking place unseen and in subtle forms around us,
darbhai is used to thwart such obstructions.
Scriptures speak of no substitute for darbhai.
The deduced fact therefore is that darbhai is the only best protection.
The primary purpose of spreading darbhai during tarpanam is to offer seat to the pithrus.
The pramana is found in the Rig hymn given below
(in which the translation for darbhai is sacred grass).
It has also been an age-old practice to sit on darbhai
and surrounded by darbhai while doing japa of any kind and purpose.
Rama did that while invoking the Lord of Oceans.
The purpose is not to lose whatever energy that one possesses or gains during the japa.
The pithrus are not to lose their energy
while descending to our plane heeding our invitation.
Hence they are seated on the darbhaasanam.
The Rig hymn is given here.
This contains the entire essence of tarpanam.
HYMN XV. Pithrus (Fathers)
1. MAY they ascend, the lowest, highest, midmost, the Fathers who deserve a share of Soma-
May they who have attained the life of spirits, gentle and righteous, aid us when we call them.
2 Now let us pay this homage to the Fathers, to those who passed of old and those who followed,
Those who have rested in the earthly region, and those who dwell among the Mighty Races.
3 1 have attained the gracious-minded Fathers, I have gained son and progeny from Visnu.
They who enjoy pressed juices with oblation seated on sacred grass, come oftenest hither.
4 Fathers who sit on sacred grass, come, help us: these offerings have we made for you; accept them.
So come to us with most auspicious favour, and give us health and strength without a trouble.
5 May they, the Fathers, worthy of the Soma, invited to their favourite oblations.
Laid on the sacred grass, come nigh and listen: may they be gracious unto us and bless us.
6 Bowing your bended knees and seated southward, accept this sacrifice of ours with favour.
Punish us not for any sin, O Fathers, which we through human frailty have committed.
7 Lapped in the bosom of the purple Mornings, give riches to the man who brings oblations.
Grant to your sons a portion of that treasure, and, present, give them energy, ye Fathers.
8 Our ancient Fathers who deserve the Soma, who came, most noble, to our Soma banquet,-
With these let Yama, yearning with the yearning, rejoicing eat our offerings at his pleasure.
9 Come to us, Agni, with the gracioug Fathers who dwell in glowing light, the very Kavyas,
Who thirsted mid the Gods, who hasten hither, oblation winners, theme of singers' praises.
10 Come, Agni, come with countless ancient Fathers, dwellers in light, primeval, God-adorers,
Eaters and drinkers of oblations, truthful, who travel with the Deities and Indra.
11 Fathers whom Agni's flames have tasted, come ye nigh: ye kindly leaders, take ye each your proper place.
Eat sacrificial food presented on the grass: grant riches with a multitude of hero sons.
12 Thou, Agni Jatavedas, when entreated, didst bear the offerings which thou madest fragrant,
And give them to the Fathers who did cat them with Svadha. Eat, thou God, the gifts we bring thee.
13 Thou, Jatavedas, knowest well the number of Fathers who are here and who are absent,
Of Fathers whom we know and whom we know not: accept the sacrifice wellprepared with portions.
14 They who, consumed by fire or not cremated, joy in their offering in the midst of heaven,-
Grant them, O Sovran Lord, the world of spirits and their own body, as thy pleasure wills it.