Supporting resumable HTTP-downloads through an ASHX handler?

Question

We are providing downloads of our application setups through an ASHX handler in ASP.NET.

A customer told us he uses some third party download manager application and that our way of providing the files currently does not support the "resume" feature of his download manager application.

My questions are:

What are the basic ideas behind resuming a download? Is there a certain HTTP GET request that tells me the offset to start at?