Bicarbonate of soda is the same thing as Baking Soda. You may know this as the thing usually sold under the 'Arm and Hammer' brand.
It is not the same thing as Baking Powder, which is baking soda mixed with some acid salt (cream of tartar) and usually cornstarch.
Why do I need it? I'm making Zombies!
Baking soda is used to foam the gelatine to make it lighter. This means prosthetics are less likely to fall off you. It has the useful byproduct of making the gelatine more porous like skin, making it easier to blend prosthetics with skin
Why is this familiar?
If you have ever made a science fair volcano, or made the plastic things inside Kinder eggs pop open explosively, odds are you used Bicarbonate of Soda and vinegar. Just like you will with the gelatine.
It can also go in homemade powdered sherbet. Possibly icing sugar, citric acid and bicarbonate of soda, but don't try it yourself without googling for a better recipe. I may be wrong :-)