If I understood Rob correctly, it is based on the number of block found in the pool the last 7 days, but if you did not found any block it check your submitted shares.
Personally I would really prefer a simpler method and less obscure, like for each block found in the pool, share the reward based on the submitted shares since last block found in the pool.
To a degree I agree; It should be based on weighted boinc-utilised shares instead of blocks.