Very large armies to construct, garrison farm alliance buildings

Sometimes I see people using very large armies to construct, garrison or farm alliance buildings, a lot larger than their march capacity. How is this possible?

For construction and garrison of alliance buildings it is possible to send more armies than your march capacity. You can click on the alliance buildings, look at the details, and use the send more troops button to send additional troops. The troops you send to construct an alliance resource building will start gathering once construction is complete, so by using a very large amount of armies to construct, you also have a very large amount of armies to gather. When timed well, this can be used to start off the gathering stage of the gold event with a lot of points.