I'm having trouble describing a fist bump explosion in my story. I can't just say "they did a fist bump explosion," because everyone may not know exactly what that looks like. Googling doesn't really help much. How would you describe a fist bump explosion in order to make it understood without going into huge amounts of detail? Thanks, everyone!
This would be a great occasion for an extended simile. Take the action and characterize the important elements by comparison to natural events. E.g. The scattering of matter after the big bang, or the bouncing of pool balls, or even a can of soda, fizzing and foaming after a fall.
As their knuckles touched, like rams crashing head on into one another [or other relevant smile], the hands opened wide, turning the muttons to butterflies, fluttering their wings to float away from each other.
It depends on the tone of your story. Is it humorous? A fist bump explosion is usually a dorky thing, so my first instinct is to describe it in a funny way. However, it also depends on the style of narrator you have. Someone like Terry Pratchett, who uses humour and often has a kind of analytical, judgemental narrator would maybe write it like this:
Despite being in their mid-twenties, they performed a silly hand gesture that was mainly popular with teenagers who did not know any better. It consisted of both participants making a fist, lightly bumping them together, and then opening the fists and moving them backwards while wiggling the fingers and imitating the sound of an explosion in slow motion. They thought it was a cool thing to do, but it was not.
If you have a more neutral narrator and only want a very basic and short description, you could go with:
They both made a fist with their right hand and lightly bumped them together in celebration. As they withdrew their hands again, they opened them and imitated explosion sounds with their mouths. They grinned at each other.
This leaves it a bit more open to interpretation while still making it quite clear what the purpose is, and of course it is left for the reader to decide whether they think this sounds silly or not. I have added "in celebration" and "they grinned" here because I wanted to explain the context, i.e. they make the gesture because they celebrate something, but it is also not a very serious gesture. (By contrast, clapping one another on the shoulder might be a more serious/earnest gesture in a similar context.)
As a side note, I think the actually important thing is not to describe the hand gesture as exactly as possible, but to get across the emotions that are involved and the context. This type of gesture is mostly done between friends, it is like an inside joke, it indicates closeness. Try to make this clear.