The first Singh Sabha was founded at Amritsar in 1873, in response to the conversion of Sikh students to Christianity and to the pressure of Hindu revivalist argument on Sikh practice. A second was founded at Lahore in 1879 with a sharper reformist line. The Arya Samaj, which came to Punjab after 1875 and whose shuddhi campaigns are often read back into this moment, was not yet a factor in 1873.
The movement's programme was schools, printing, standardised religious practice and the removal of what it identified as Hindu accretion from Sikh ritual. It produced Khalsa College at Amritsar in 1892, a network of Khalsa schools, and a large body of Punjabi print.
Its intellectual centre was Bhai Kahn Singh Nabha, whose Ham Hindu Nahin, 1898, argued that Sikhi is a distinct religion, and whose Mahan Kosh, 1930, is the encyclopaedia of Sikh literature and remains the standard reference.
The Tat Khalsa faction of the movement won, and the practice it defined, the Anand Karaj marriage rite given legal force in 1909, the removal of idols from the Harmandir precincts, the standard rehat, is Sikh practice today.
What it displaced, the Sanatan Sikh tradition with its Hindu ritual overlap, its Udasi and Nirmala scholarship and its caste accommodations, was the majority practice in 1870. That is a very large change made in about fifty years and it is usually described as a recovery rather than as a change.
It began at Amritsar in 1873 and at Lahore in 1879, and the two wings differed on whether Sikhs were a distinct community and on how far reform should go.
It built the schools, the colleges, the presses, the tract societies and the dictionaries, and it produced the standardised practice that the Rehat Maryada later codified.
It also narrowed things. Practices it regarded as accretions, shrine worship, the jatheras, the sant traditions and a great deal of local custom, were pushed out of respectable Sikh practice and mostly continued anyway.