What is double-acting baking powder?

Answer 1

it contains two different types of acids that react at different times. The first acid will react by creating gases when mixed with the liquid in the recipe. The second type will react by creating gases when the batter is exposed to oven heat.

Answer 2

Double acting baking powder gives two different chemical reactions which help baked-goods to rise. The first reaction occurs when the the double-acting baking powder comes into contact with a liquid (a neutralization reaction which releases carbon dioxide), the second reaction occurs when the batter is then heated (i.e baked). This makes it particularly suitable for if you wish to make a batter/dough up in advance and bake it a while later, since it should mean that your baking will still rise sufficiently.

"Normal" (single-acting) baking powder reacts just when the liquid is added, so if it is used in baked goods they should go into the oven the minute the batter/dough is complete, otherwise rising may be affected.

Unless the tin specifies "double acting", you should assume it is just single-acting baking powder.