Straight from the OP wiki
Don (ドン Don?) is a sound effect used in manga. In anime format, this effect often sounds like a loud "boom" or drum beat. It is often used to emphasize something happening.
An example of the usage of "Don": a person appearing on the scene, the scene pauses for a moment while the character's name appears and as the text for the name comes on the screen the sound effect "Don!". By adding the "Don!" effect, this implies the person is very important to the storyline.
is also an english phrase of similar meaning: Do it with style
The Franky flashbacks arent bs though either they are important. It provides information on what kind of weapon Pluton actually was and how Franky came to have the blueprints in his possession
