What does a buck converter do with the energy that a linear regulator would otherwise waste as heat?
Yes, crudely put, it converts the voltage into current.  Ignoring the internal losses, if you halve the voltage you double the (available) current.

So if you convert a 5V 1A input into a 2V output the current capacity becomes 2.5A minus internal losses.

Say you have a switching efficiency of 85%, then you can work it out as:

$$
\frac{5V}{2V} = 2.5 \times 0.85 = 2.125
$$
$$
1A \times 2.125 = 2.125A
$$

You can think of it as the DC equivalent of a transformer for AC.