Ok ill try to explain, its about re-opening the betting. The all in re-raise must be double the previous bet in order to be a valid bet to re-open the betting.
Example. Lets say the blinds are 500,1000
Tenbob- 256,590 in chips
Robwhufc- 90,000 chips
Chuck-T- 1,500 chips
Chuck T posts BB.... 1000 (leaving 500 chips)
Rob Posts SB... 500
Tenbob CALLS 1000
Robwhufc CALLS 500
CHUCK T Raise all-in.......... becuase the min raise in this situation to re-open the betting MUST be double the origional bet of 1000, then the betting is not re-opened, and the limpers only option is to call. They can then play a side pot on further betting rounds.
Hope this answers your question, i always find this one tricky to explain, i know there is an article on cardplayer somewhere in the archives about it.