WE PROPOSE 12 OBSERVATIONS OF A BLACK HOLE TRANSIENT OF 3.6 KSEC EACH WITH NICER TO BE MADE SIMULTANEOUSLY WITH VLT FAST-TIMING MEASUREMENTS IN THE INFRARED. THESE DATA WILL BE USED TO UNDERSTAND THE EVOLUTION OF THE IR/X-RAY CROSS-CORRELATION FUNCTION THE LAGS IN WHICH GIVE FUNDAMENTAL INFORMATION ABOUT THE SPEED OF THE RELATIVISTIC JETS IN THESE SYSTEMS. NICER WILL ALLOW HIGH THROUGHPUT AND ACCESS BOTH TO THE GEOMETRICALLY THIN AND GEOMETRICALLY THICK SPECTRAL COMPONENTS IN THE ACCRETION FLOW. BY OBSERVING THE EVOLUTION OF THIS CROSS-CORRELATION FUNCTION WE WILL BE ABLE TO MAKE THE FIRST OBSERVATIONAL STUDY OF THE EVOLUTION OF THE JET SPEED IN X-RAY BINARIES.