Originally Posted by cubewheels

If you use your core muscles to unload your arms durind downhill coasting, it will also unload the saddle by over half the weight.

If you simply pushed on your feet to try to take some of the weight off the saddle, you may or may not relieve some of the pressure from the arms so using the core muscles is a better approach to the same goal. This also requires adjustments to the bike's setup to work - you must be able to take your hands off the bars in the normal riding position without falling backwards or forward, perfectly balanced.

https://youtu.be/gbFrz5FsIow?t=488