RTsync Blockset extends MATLAB Simulink simulation capabilities in real-time simulation. Blockset consists of a single block which, when placed to a model, syncronizes a simulation time to CPU real-time clock.
A concept of real-time simulation in Simulink is rather simple buf effective. Normally, model simulation runs much faster than real time (if simulation runs slower then no one can help, not in Windows OS). It is possible to measure a time of each simulation step and actual CPU time taken. So you can calculate a time delay a Simulink process must sleep after each simulation step to perform real-time simulation. As a result, you achieve a real-time modeling with no additional CPU load. RTsync Blockset utilizes theory mentioned above and some other magic.
Actually, RTsync Blockset does not make a breakthrough in real-time simulation. There were several tools and ideas for real-time simulation in Simulink before. MATLAB developers also propose some methods. Personally, I have used Real-Time Blockset for Simulink by Leonardo Daga for couple of years before I found an inexactness in it in some cases and wrote my own for better simulating results.