Yudane is a scalded flour paste used in soft breads: boiling water is mixed into part of the formula's flour, then the paste is cooled before the dough is made. The cooked starch helps retain moisture. Many formulas use equal flour and boiling-water weights for the scald, but the recipe governs the proportion.
Unlike tangzhong, which is stirred and cooked with more liquid, yudane uses the heat of added boiling water. Do not substitute the pastes without adjusting the formula.
Reference: Further reading.