He ordered that 'zakat' (obligatory payment under Islamic law) would be deducted directly from bank accounts.
The Shias refused to abide by it and protested en masse, forcing the general to exempt them from the deduction. This led to the consolidation of Shia identity as a person had to declare that he was Shia to ensure that zakat was not deducted.
Sunni sectarian outfits gained a lot of ground during General Zia's rule. Saudi and Iranian funding to respective sectarian outfits have allowed them to thrive through the subsequent decades.
It is essential to understand the dynamics of sectarian violence in Pakistan. Although estimated to be around a fifth of Pakistan's population, the Shias have had disproportionate share of power.
Mohammed Ali Jinnah, Nazimuddin, Mohammad Ali Bogra, Iskander Mirza, Zulfiqar Ali Bhutto, Gen. Yahya Khan and Benazir Bhutto were all Shias, and even today the president and the chairpersons of both houses of parliament are Shias. Generally, Shias are better educated and even richer. Many of them are successful lawyers and doctors.