(Though the question may seem long, actually it is short. Even if one is able to read till the first point of Edit it would be nice)
It is seen that while performing the last rites (antyeshti, pindadanam, etc) and shraddha tarpanam rituals for ancestors in accordance with the Karma Kanda, the manner in which rituals are done are different from those at the time of worship of gods. For example-
1) The yajnopaveetham (janeu, poonool) of the performer is kept on the right shoulder, the technical term for which is apasavya. Instead of keeping it normally on the left shoulder. Even during the tarpanam, while chanting the mantra for the devatas, the yajnopaveetham is kept on the left shoulder but before chanting for the ancestors, it is shifted to the right.
2) Chandanam (sandal) is applied using the index finger, instead of the customary ring finger
3) Water, sesame seeds rice balls (pindams) are also offered turning the hand towards the thumb side.
4) A ring of Kusha (darbha) grass is tied in the thumb and finger, not normally done for devata worship.
What is the reason behind performing these rituals in such a different, almost opposite manner?
Is it related to a boon by any god or anything concerned with energy flow?
Even if someone knows the reason behind a few please do share, especially the yajnopaveetham point, it would be gratifying.
EDIT: I recently read that different places in the palm have been allotted for different purposes:-
1) The upper portions of all four fingers are called as Dēvatīrtha, all offerings to devas are thus given from there.
2) Similarly, the area between the thumb and index is called Pitrutīrtha, and is used for tarpanam.
This division seems to have been mentioned in the Manusmriti 2.59:
अङ्गुष्ठमूलस्य तले ब्राह्मं तीर्थं प्रचक्षते।
कायं अङ्गुलिमूलेऽग्रे देवं पित्र्यं तयोरधः॥५९॥
The part of the palm at the root of the thumb they call ‘the receptacle dedicated to Brahmā’; that which is at the root of the finger ‘Sacred to Prajāpati’; that on the finger-tops ‘dedicated to the Gods’; and that which is below these two ‘dedicated to the Pitṛs.’—(59)
EDIT 2: There is a reference to such a division in the Vaikhānasa Grihyasūtra Praśna 1 Khanda 5 as well:
Of the right hand, the middle part of the palm is the part sacred to Agni ... the part between the index and thumb is that sacred to the Fathers... An act relating to Gods he performs with the tīrtha of the Gods... one relating to the Fathers with the Fathertīrtha...
Other websites claim that this information is also mentioned in the Mahabharata, Linga Purana, Devi Bhagavata, Bhavishya Purana (not been able to verify where).
I couldn’t find the reason (as asked above in the question) for the area between the index and thumb being allotted as Pitrutīrtha and why that area is used for Pitru tarpanam. Hope this information helps in further research.