Something seemed to get cut out of that answer....
You have this huge network of condition fragments, and when a new piece of data (a record) comes in, it trickles through that network. Sometimes that piece of data finds that it and some data that has already entered the system completely satisifies the IF clause of a rule. When that happens, the THEN clause is added to the agenda. The THEN clause can access the specific records that matched the IF clause. Sometimes a piece of data results in many many rule matches being added to the agenda. Sometimes a piece of data doesn't satisfy a complete rule, and when that happens, it remains in the network in the queries that it helps partially match.
