While tying the rakhi on her brother's wrist, a sister may recite:
येन बद्धो बली राजा दानवेन्द्रो महाबलः। तेन त्वामपि बध्नामि रक्षे मा चल मा चल॥
(Yena baddho bali raja danavendro mahabalah, tena tvamapi badhnami rakshe ma chala ma chala)
The mantra refers to the sacred thread with which the powerful King Bali was bound. In simple terms, the sister invokes the same protective spirit while tying the thread around her brother's wrist and prays that the bond of protection remains firm.
येन (Yena) = By which
बद्धो (Baddho) = Bound
बली राजा (Bali Raja) = King Bali
दानवेन्द्रो महाबलः (Danavendro Mahabalah) = The great king of demons, who was very powerful
तेन (Tena) = By that same thread
त्वामपि (Tvamapi) = You too
बध्नामि (Badhnami) = I am tying
रक्षे (Rakshe) = O Raksha / protective thread
मा चल मा चल (Ma Chala Ma Chala) = Do not move, do not go away from your vow of protection
The Raksha Bandhan matra thus translates, "The thread by which the mighty demon king Bali was bound, I tie that same thread around you. O Raksha thread, stay firm and don't move away from your vow of protection."
The phrase “मा चल, मा चल” is traditionally understood as a prayer asking the protective thread to remain steadfast.