In the above GIF from Big Brother (2021), Derek F., Azah, Tiffany, and Hannah are experiencing extreme joy; so much so, that their bodies, literally, couldn't bear the weight of the emotion.
Derek F. braces himself on the designer sink. Azah and Tiffany drop to their knees. And Hannah looks quickly for a place to sit before plopping onto the bed.
But interestingly, the body can react to joy and misery in similar ways. It's not uncommon to see people react to, say, the news of a deceased loved one by plopping, bracing and/or dropping to their knees. The main difference is that misery is often coupled with crying while joy and laughter are often couples.