MrPlow - The difficulty used for a new mining session and also the lowest difficulty allowed. Defaults to 22, meaning a miner has to do on average 2^22 mining hashes to get a share. If the miner gives shares too fast with this difficulty, then the difficulty is increased until the mining rate is reasonable.
Reasonable is defined as between 4 and 12 shares submitted every 120 seconds. Difficulty for a session may also be adjusted down automatically, but never lower than the min_diff.
NOTE: CHANGING THIS VALUE ON AN EXISTING MRPLOW DATABASE WILL CAUSE THE VALUE OF EXISTING SHARES IN THE DATABASE TO BE WRONG.
This is because the database storage assumes 1 share equals 1 share a min_diff so if min_diff is changed the value of the existing shares will be wrong. However, it will be roughly correct and will straighten itself out in a few blocks. The all your miners are the same before and after a min_diff change, there is no impact on their rewards. If a new miner starts or increases right after a min_diff change or an old miner drops off, they will get skewed amount for a time.