I'm guessing: the burning turns the sugar into carbon plus oxygen and hydrogen as water vapour. The bicarb helps to soak up the water to leave the carbon to bind to itself, making the weird ash column.
This is a really good description of how our economy works over large and small periods of time. Really worth giving half-an-hour to watch it! Even Gabe watched it with me, and we've been discussing where money comes from and how it can be used since. (Thanks +Cobi Smith) How The Economic Machine Works by Ray Dalio