You can think of your sonohystogram and sounding procedure as a mock embryo transfer. The purpose is to take a thorough look at your uterus and make sure that there aren't any fibroids or anything else blocking the cavity and impeding the transfer. Because of the extreme time-sensitivity of IVF, your doctor wants to make absolutely sure that when it comes time for your embryos to be transferred, your uterus is able to receive them.
The doctor will insert a small catheter through the cervix and into the uterus. With an ultrasound, he will use small gradations on the catheter to determine the depth that is best for transfer. There is a precise location where the embryos should be deposited; knowing the depth at which to place the catheter will make your transfer go much smoother. He will then instill a little bit of sterile water into the uterus to help him see the cavity and make sure there is nothing occluding it. He'll also take a measurement of your uterus, which tells him exactly where to put the embryos on transfer day.
The cervix is so narrow, or stenotic, in some women that the catheter cannot get through. While this is easily fixed through a very minor surgical procedure, you don't want to find this out on the day of your transfer.
You may notice a small amount of cramping during the procedure. If your cervix is particularly narrow, that cramping may be more intense. If you are concerned about the pain, take 1000mg of Tylenol (two extra strength tablets) about two hours before your test. The cramping should dissipate once the test is finished, usually in a matter of minutes.