Page 1 is blank. Ten pledges at 0.01 ETH fill it, and one of them is drawn to take the page: 0.1 ETH. Line 1 is open.
The first full page is drawn here, its winning line circled.
The tenth pledge fixes a block three ahead. Its hash draws the line.
Holders of 2,000 $PLEDGEBOOK sign, up to three lines a page. Each line is a 1-in-10 shot at 0.1 ETH.
Ten pledges at 0.01 = 0.1 ETH. Page 4 fills on block 1,000, so the draw block is 1,003. Its hash, run through keccak256(hash, 4) mod 10, gives 6: line 7 (counted from 1) takes 0.1 ETH, its own 0.01 plus nine pledges of 0.01. Sign three lines and you hold three of the ten chances.
Nobody. The tenth pledge fixes a block that has not been mined yet; the contract reads that block's hash and the line falls out of it. If the hash ages out before anyone settles, the contract fixes a fresh future block. Anyone can recompute every draw from the tape.