The original paper:
https://bitcointalk.org/index.php?topic=3165.0In the beginning, all pools were proportional - when a block was found, pools immediately paid out the block reward to miners based on their contribution since the start of block hunting. If a block was quickly found, then miners would get higher earnings per amount of work submitted. The strategy is to "hop" to a new pool as soon as they find a block and start a new round of block finding, hoping for an early block find, then abandoning that pool as soon as another pool starts a new round. By doing so, you are investing your mining work in pools only when there is an above-average reward potential.
It is bad because it reduces the income of full-time miners - they get less work submitted during the profitable times and are left mining alone on long blocks.
