If you own the Nikon SB-400 speedlight, repeating flash option is not available for your use with the Nikon D90. In those instances where you see its use, you have to rely on the built-in flash to get the job done.
There's some serious logic you need to understand while in the "Flash Cntrl for built-in flash" menu (e2) to use repeat flash. First, we determine its power output for each repeating flash - between full and 1/128. With that done, the Nikon D90 will automatically limit you to determine your desired number of times it fires. (Note - your desired value may not translate to actual number of times the flash fires). Take note that in its full capacity, the built-in flash can only reach up to 18m at ISO 200 / 20 Degree Celsius).
Hence, at say, 1/4 power, you can effectively reach subjects not further than 4.5 meter away from your camera. With that in mind, the power of the repeating flash you set is directly linked to the distance of your subject, which in turn, affects the total number of times your built-in flash can fire in one second!
Now, its quite sensible to set your shoot mode at shutter priority for repeating flash shoot. Hence, if you set your repeating flash to fire at 5 times per second, you should get five "ghost" images of your subject in motion during shutter release duration of that one second. Try it with your kids or a moving object.
My kids love it!