If the round duration at the end of the round is greater than 1 hour, each user is looked at to see their 'window of participation', which is the time difference between their first and last submitted shares.

If the time difference between their shares is less than one half of the round duration, their share count will be reduced by 50% and the other half of their shares will be credited to an account setup by the pool operators.

We've monitored this over the past 10 rounds and seen no higher than $0.75 BTC worth of shares removed per round from pool hoppers, and on average, less than 15 accounts are affected per round.

We believe that this will become an effective deterrent for pool hopping, as it will remove the benefit of doing so, add incentive to stay participating in the pool, and at the same time, offer FairUser and Myself a minimal compensation for all of our hard work and late nights.

We considered our options and alternatives with this for quite a long time before deciding to implement it and we believe it is a fair method that allows an easy correctable course of action for each affected miner to take.

Edit: (Posted this below, but added it here too)

I wanted to clarify exactly what this is doing...

As an example, lets say the current round has been going for 2 hours when we solve a block. Our framed window for checking on pool hoppers will be 1 hour.

Each user will then have timestamps taken of their first and last share of the round to see if the difference between them is greater than or equal to 1 hour. If it is not, we will assume they're pool hopping and we will discount their shares by 50%.

It doesn't matter if the user joins 30 minutes into the round and leaves 30 minutes before it ends, as long as they were present for a minimum of 50% of the round duration, they will not be affected.

On average, this affects 5 - 10 users per round.

This does not make shares submitted later in the round have more value.
This does not favor GPU over CPU miners.
This does not affect any user if the round is shorter than 1 hour.
This is not a score-based system.

Awww yeah, it's like Christmas all over again.  Got 500 GH/s coming your way... at least for 42% of the time.

We have decided to clear balances of all users who have the balance turned negative after the withdrawal of mistakenly earned coins. Despite the fact that the balance has been calculated correctly. This was done as thanks for participating in the pool. In sum, we gave users 80 BTC.

I think he meant in the introduction to the DeepBit thread (ie this thread).