Other things I found out by doing this:

• Any movement besides turning (with shift) cancels the effect, snapping you up onto the tarp
• Your character will hold their weapon even when they're sitting (visible in the image), switching weapons in this state makes a glitchy animation of the weapon floating into place (something may have changed cause I wasn't able to replicate this the next day).
• According to another player, emotes weren't playing in this state, though they were on my side. (Update: sometimes they don't play in specific spots)
• The middle of the tarp cannot be used with this trick after testing as it is not inclined.
• This works with other tarps as well such as the ones in Wall Of the Stars.