In Huggin's book "It's All In Your Head", he explains this.

He said when all fillings were removed in a patient except one, urine mercury stayed generally the same, but when the last filling was removed, most patients urine mercury went through the roof.

In short, he said this indicates that there is some mechanism in the body that senses that mercury exposure is continuing and therefore, the body does not start dumping mercury. But when the exposure stops, the body begin dumping mercury usually within 24 hours.

He said the mechanism that does this is unknown, but it is fairly consistent in about 80% (if I remember correctly) of patients.