Using kiwi fruit paste is the best way to tenderize tougher cuts of steak. Actinidin in the fruit breaks down the connective tissue in the meat without turning it into pulp.
Kiwi has a relatively neutral flavor compared to other fruits that contain actinidin like mangos and pineapples. To use it as a tenderizer for meat, puree a kiwi fruit and add 1 to 2 tablespoons per cup of marinade. How tender the steak turns out depends on the amount of kiwi fruit used and how long the steak is allowed to soak.
Mechanically tenderizing the meat with a mallet or other tool also breaks down the connective tissue but breaks down the fibers of the meat itself as well. This can leave steak in a mushy state, though for recipes where the goal is to flatten the steak as well, it is the best option.
Thermal tenderization causes the collagen in the meat to turn into a gelatin at high temperatures which causes the muscle fibers to be surrounded by a gel. This replaces any moisture that might be lost during cooking and gives the steak a juicy, fork tender texture. Thermal tenderization is a slow process, however, and takes hours or days to achieve.