Traffic Simulator and the built in Diagnostic Logging capabilities are two new features included in ISA Server 2006 SP1. Today Dr Tom Shinder released the part 2 of the deep dive explanation about those features. I strongly recommend you to read both articles and get the full understanding of how each feature works.
Your New ISA Firewall: ISA 2006 Service Pack 1 (Part 1)
Your New ISA Firewall: ISA 2006 Service Pack 1 - Part 2: Traffic Simulator and Enhanced Diagnostic Logging