1. open the Eden editor, place a random soldier with a weapon and preview the mission
2. get in the high prone stance - press CTRL+S until the character is sitting
3. press A, S or D (left, right or backwards) until the hand animation (pushing) starts and just let the key go
4. the soldier magically transists over to the prone position (legs will clip through the ground without any collision)

Best visible if you activate the 3rd person view.

It does not work, if you shortly trigger one of the direction keys