It's actually fairly easy to use gfxCardStatus. (https://gfx.io/)
1. Download and launch gfxCardStatus.
2. Click on gfxCardStatus' menu bar icon and select "Integrated Only"
3. Launch JDownloader.
If JDownloader (or another app that uses the discrete graphic card) is already launched, the switching won't work (gfxCardStatus will tell you anyway).
Here's a link to the screenshot: http://i.imgur.com/vFP3GTI.png
Thx to Eric C.
