ToggleDrillState changes the default drill state from a returned set – so if the first query returns a member in a hierarchy rolled up then it will drill it down, or vice versa.

For example,
{[Product].[Customer Lending].Children}
Returns:

Whereas this query:
{TOGGLEDRILLSTATE( {[Product].[Customer Lending].Children},{[Product].[Demand Loan]} )}
Returns: