IIRC, the Gospel accounts said that the soldiers broke the legs of the 2 thieves to hasten their deaths, but when they came to Jesus they saw that he had already died.
You are correct. It is a practice called crucifragium. It is intended to prevent the victim from raising himself up on his legs to be able to breathe, which will hasten death due to asphyxiation. Basically without being to lift up by pressing down on the nail through the feet, the victim cannot get enough expansion space in the lungs to exchange air, due to the arms pulling the abdomen abnormally upward. CO2 builds up in the lungs and then damage to the lungs start a fluid build-up, further reducing lung capacity. Shock from the pain of broken legs also adds to the rapidity of death as well.