The Dreamcast Puru Puru Pack (referred to as the Dreamcast Vibration Pack in Europe and the strangely named "Jump Pack" in the US) is an accessory for the Dreamcast controller's expansion ports. When placed in the controller, it allows compatible games to give force feedback to the controller, a concept pioneered by Nintendo with the Nintendo 64. Although. unlike the N64 version it runs of of the power of the console so doesn't require batteries. These were the days when controllers didn't have built-in vibration as standard and we'd have to shell out some cash just to feel something in a game (although Sony would soon go on to address this issue with release of the PSone).
The accessory is bulkier at the back due to the location of the motor meaning the ideal position to place it is in the second second expansion slot. Placing it in the first is possible although it wont lock into place like it would if inserted at the back. It can also be placed inside the Dreamcast Light Gun to give the controller a more realistic feel when firing. When you insert both the VMU and the Puru Puru Pack the controller feels sturdy and possibly a little heavier than most controllers of the day but probably equivalent to today's modern featured packed joypads.
My favourite use of the accessory has to go to the psychedelic shooting game Rez, without it, the game is quite a static experience but with a Puru Puru Pack inserted, you can feel the pulse of the beat of the level, further drawing you into the experience.