Mike,
The "reinitialize" input in the PID Control Toolkit, in general, will load the initial value or make the internal states zero for the specific function. The online help (press CTRL-H over the function and go to "Detailed Help" or "Click here for more help") will exactly tell what will happen for each function used in the PID Control Toolkit.
Hope this help...
Barp
Control Design and Simulation Group - National Instruments
Barp - Control, Simulation, RTT and HIL - National Instruments