Finally, it inverts the final part of the expression. The inversion of greater than or equal to is less than, so that's what Sequel changes.
Just like datasets, Sequel uses a functional approach when modifying the conditions, so what really happens is that all nodes in the abstract syntax tree that need to be modified return modified copies.
So you can use conditions in one query, invert the conditions for use in another query, and have both queries work the way you expect.