Last Saturday I shot Kodak Tmax 400. I pushed the film 2 stops, shooting it at 1600. There was not enough light to shoot the scene at ISO 400. So, I metered the scene at 1600 ISO. When you push film by 1 stop you are allowing the film to use half of the light it was originally intended for. ISO 400 becomes ISO 800. When you push it 1 more stop now you are at ISO 1600 and have again allowed the film to use half as much light again. Now there is enough light for me to take the shot because I shot the image at an appropriate ISO for the light available. There is a trade-off in image quality which you try to mitigate in the development of the film. Nikon F4 with Voigtlander 40mm Lens at F8. Kodak Tmax 400 pushed to 1600 and developed in Tmax Developer 1:4 for 8 minutes 30 seconds.