Normally, a browser will establish a single network connection with a server for a file transfer. Download Wizard can try to "segment" the file into multiple pieces and download them individually but at the same time. This can, in many cases, significantly decrease the amount of time it takes to download the file. Keep in mind that many people configure their servers to prevent this sort of behavior, so your mileage may vary. In order to both allow the acceleration feature but try to remain a good "network citizen", Download Wizard currently only supports a maximum of two simultaneous connections for download acceleration.
Enabling it is as simple as going into Preferences and turning on the checkbox under the "Advanced" tab.
Note: For broadband users, download acceleration can make transfers faster in many cases - but not all. It will sometimes make no difference and in some cases can cause problems if servers are configured to prevent it. For dial-up users, it will make no difference, and in many cases will slow things down. Please keep all this in mind when using this feature.