If you were trading the SPs on Wednesday, you probably noticed a marked chance in behavior re the "noise level". 10 ticks up, 10 ticks right back down, 10 ticks up, ten ticks down...., also, affectionately known as "noise". You probably also noticed a huge drop in the size of the bid/ask. All markets have their own footprints and the footprint in the SPs had definately changed mid week. It is pretty hard to keep things "mum" in this industry, so it did not take more then a few hours of networking around to learn that one of the largest firms specializing in algorithmic "black box" auto systems (market making), had taken a rather large hit the prior few days, and had turned off all their auto-trading systems. Another large firm specializing in the same game, claimed "technical difficulties" and had also shut off their auto trading systems. These type of systems add liquidity to the system and generally make up about 40% of the volume in the e minis. Some of the systems work in the manner that a floor specialist would - buying on bid, and selling out on offer 2-3 ticks higher, or shorting on offer, and covering on bid 2-3 ticks lower - constantly extracting dollars out of the spread between the bid ask.