Pre-tensioning involves applying tension to steel tendons before casting concrete. The tendons are anchored and tensioned using jacks, then concrete is cast around them. After the concrete reaches strength, the tendons are cut loose, transferring their prestress to the concrete. Key stages of pre-tensioning include anchoring tendons, applying tension with jacks, casting and curing concrete, and cutting tendons once concrete has gained strength. Special equipment like prestressing beds, molds, jacks, anchoring devices, and harping devices are used in the pre-tensioning process.