It is probably better thought of as “dried-frozen”, rather than freeze-dried tofu.
It swells to twice its size when rehydrated.
Commercial brands usually have an expander added to the tofu while it is being made, which helps it to soften up better. They call this “kansui”, which is usually potassium carbonate, potassium phosphate, sodium carbonate or phosphate.
The dried tofu can be ground into tofu meal and flour, or made into tofu croutons. However, it is usually simmered in dashi. After simmering, it will have a spongy texture.
Most Koya Tofu (90%) is made in Nagano Prefecture of Japan.
Rehydrate Koya Tofu in warm water for about 5 minutes each side before using. Cover with a weight such as a plate to help keep it down in the water.
Then drain, plunge into cold water, let sit for a bit, then press until somewhat dry (watch for any very hot water that might come out.)
Then place in clean water, let it swell again, then squeeze again.
Some cooks like to repeat this final step several times.
Koya Tofu was probably first made in China.
The name comes from Mount Koya, where there was a Buddhist temple. The monks would cut tofu into thin slices, and put it outside to freeze. Then they brought it in, thawed it, pressed it lightly, and dried it in warmed sheds.