I remember hearing about hybrid pools in the past and thinking they were a good idea, but I forget how they are intended to work. From what I recall, it is a set of p2pool "supernodes" basically. Is that correct?
More simplistically it is just a "normal" pool where the miner is smart. Like in p2pool the miner chooses it own transactions. How it differs from p2pool is you still have a traditional pool front end to handle rewards, manage accounts, check stats, etc.
A hybrid pool could integrate into p2pool being a super node to reduce variance (for both the pool & p2pool).