Shaddows - 8:46 am on May 19, 2011 (gmt 0)
Traffic throttling- Description
Typically means you have a ceiling level of traffic. It comes in two varieties- total traffic and Google traffic. Once you hit your traffic allocation for the day, you get no more. Usually it's a bit more subtle- specific referrals taper off, or get replaced by those with a lower search volume.
Classic Symptom: "Buzz Saw effect" when you view your daily traffic. Especially evident when you get a traffic spike for one referral, only to see a drop for other key referrers.
NOTE: Best seen from headline stats. The more you drill-down, the less you are sure of what you are seeing
Traffic Shaping - Description
Typically two things happen simultaneously. First, Google starts showing your pages to people with different user intentions (or just turn off user-intent filtering). See Symptom (a) below.
Secondly, Google slips your site into SERPs that it did not appear in before, while removing it from others. See symptom (b). Actually, what they do is some subtle re-ordering while rotating Universal Search features through other positions, affecting your exposure.
a) Traffic on normal referrals exhibits strange behaviour. The oft-reported Zombie Traffic just will not convert. Potentially, other terms start converting better.
b) Referral patterns change. So the traffic coming from google is arriving in unusual proportions when broken down by keyword.
NOTE: You cannot see this effect in your headline stats
KEY OBSERVATION FOR BOTH
Perceived rankings do NOT change significantly- apart from possibly a yo-yo on a SMALL number of terms. Also, there is nothing to stop a hybridisation of these two tools, keeping parts of the site in "normal" conditions, while "testing" others