Preparation of octopus

It may not be done at home, but before cooking, octopus legs are rubbed with grated daikon radish and then pounded with an appropriately cut daikon radish.
By doing this, dirt and slime will be removed, and at the same time, the body will become soft. (However, if you beat it too hard, the skin will not come off when you boil it.)

Daikon radish contains various enzymes, the most active of which is amylase (diastase), which breaks down starch. It also acts as a protease that breaks down proteins and lipase that breaks down fats.

If you don't have daikon radish, or if you're going to boil it, you can wash it with salt because it doesn't matter if it's salty. However, it is easy to get tight, so you have to wash it quickly.
In addition, rice bran, green tea, bancha, and pu-erh tea are sometimes used instead of grated daikon radish and salt for washing.
apart from the above
(1) Freeze the raw stuff once, and soften it after breaking the cells and tissue structure by freezing (From Sushi Science)
(2) Loosen the muscle tissue by rubbing vigorously with salt or grated daikon radish (From Sushi no Science)
(3) Wrap the washed octopus in a towel and tap it with a bottle (From 100 Ingredients and Japanese Cuisine)

*You don't have to make it soft, just to enjoy its texture.
In addition to octopus, when steaming or boiling abalone or turban shell, add grated daikon radish or scraps of daikon radish to make it softer.
In addition, there is also a technique to soften it by steaming it with sugar instead of grated radish.