Meaning: When an elderly person arrives, the vital energy of the young person starts rising and when he gets up and does Namaskar, it returns to normal. - Manusmruti 2.120; Mahabharat, Udhyog, Chapter (Section) 38.1, Sr. no. 104, 64-65
Explanation : 'As the sojourn of the elderly person is gradually towards the southern direction, that is, towards the region of Lord Yama (towards death), his body starts emitting raja and tama frequencies on a high scale. When such an elderly person comes in the vicinity of any younger individual, these frequencies start affecting the younger person. A subtle magnetic field is created between the two. Consequently, the vital energy of the younger person is pulled upward. This way the younger person can suffer due to sudden momentum to his vital energy. When this younger person does Namaskar to the elderly person, some amount of the Central channel of his Kundalini system is activated and the sattva component in it starts increasing. Consequently the raja and tama components in him are influenced by the sattva component and the vital energy comes back to normal state. Hence on arrival of an elderly person, it is customary for the younger individuals to do Namaskar to them.'
When travelling, prior to the commencement of a journey and upon returning, why should one do Namaskar to elders in the family?
'Namaskar to the elders in the family is one way of surrendering to the God principle in them. When an embodied soul bows in Namaskar to an elder by surrendering to the God principle in him, at that time a sense of compassion is created in his body. This compassion percolates right upto his subtle body. At that time, energy of his mind is activated and in turn activates the five vital energies, which are located at the seat of the Manipur chakra (situated in the Naval region). Transmission of these five vital energies all over the body then awakens the soul energy. With the strength of the soul energy, the Central channel gets activated and converts the expressed energy of spiritual emotion to the unexpressed energy of spiritual emotion. With the help of this unexpressed energy of spiritual emotion, the embodied soul, through the medium of elders, gains the required Deity's principle from the Universe. For this purpose, while leaving the house on a journey, the embodied soul should do Namaskar to elders and with the strength of sattva frequencies has to protect himself from distressing frequencies in the atmosphere. Similarly, returning from a journey, one should immediately do Namaskar to elders and awaken the God principle in them, which would disintegrate the raja-tama particles from the air around him, which might have been brought along.
(Ref: Sanatan's Publication - 'The Correct Methods of doing Namaskar')