jump flash (or joke flash, screamers, etc) are created with the sole purpoe of making the viewer concentrate on the screen with objectives like: "find 8 things wrong with this room", "Wheres Waldo?", or ask to listen to something. Then while the user is focused on the screen, a scary image is to quickly come up with a high pitch scream. The user is then intended to either 1. jump from being startled, or 2. shit their pants. While this was widely popular, it could be that you didnt know if the flash you were watching forreal, or a jump flash. It even apperared on Starcraft (a very kick ass game)online, where you had to find some things wrong with a base, and then it would move you to a black area of the screen showing a text face with a scream. it even told you to turn up your sound to hear clues. Nowadays, they are no longer as popular anymore. people can reconize one from a mile away as the author would give away clues to show what it was (turn up your volume, focus real hard, etc)and most people dont even jump anymore. Such flashes are still on websites like ebaumsworld or Albinoblacksheep, who has a section just for them. Other sites like Newgrounds are smart, and makes sure this overused crap dont get in the portal.
Person A: hey man, watch this really cool flash i found
Person B: ok
A: you have to cocentrate as much as you can
B: im looking and listening real hard but i dont see anything
*Image and scream happen*
*A starts laughing his ass off*
*B kicks A's ass*