TL;DR
According to the Sau Sakhi (100 Sakhian), Sri Guru Gobind Singh Ji promised that if a Sikh ever falls into hell, he himself will reach down and pull them out by their kesh.1
The kesh is the Guru's grip on the Sikh. So long as the Sikh keeps it, the Guru does not let go.
The sakhi
In the Sau Sakhi, Sri Guru Gobind Singh Ji tells his Sikhs that even if they fall — through weakness, through circumstance, through the burdens that come to every soul — they need not despair.
So long as they have kept their kesh, the Guru himself will not abandon them. He will reach for that kesh, the visible mark of his Khalsa, and draw them up out of any depth they have fallen into.
This is not a transaction. It is a covenant. The Sikh keeps the form the Guru gave; the Guru keeps the Sikh.
What the kesh actually is
The hair on a Sikh's head is not a personal style choice. It is the Guru's mark — given at Vaisakhi 1699 when Sri Guru Gobind Singh Ji established the Khalsa and made kesh one of the Five Kakaars.
Kesh is like the umblical cord, just like a mother provides nutrition, similarly Guru bestows boons to their children in this material world.
ਵਾਹਿਗੁਰੂ ਜੀ ਕਾ ਖਾਲਸਾ ॥ ਵਾਹਿਗੁਰੂ ਜੀ ਕੀ ਫਤਿਹ ॥
1. The Sau Sakhi (100 Sakhian) is a traditional collection of stories attributed to Sri Guru Gobind Singh Ji. It has been carried in Sikh oral and written tradition for centuries and is cited regularly in gurdwara katha and family teaching. Its textual history is debated within academic Sikh scholarship; this post presents the sakhi as Sikh tradition has carried it. ↩