From the Adorama YouTube Channel:
In this episode Mark Wallace demonstrates three ways to add light to interior room to match the bright light from the sun. After watching this tutorial you'll be able to balance the light so your interior shots look natural with the light from outside. Mark shows how you can use a speedlight, a small off-camera flash, and even a fully manual studio setup.
Note: Instead of using high-speed sync, you can also use ND filters to reduce the shutter speed required for a proper exposure to arrive at a shutter speed at or below your camera's max flash sync speed. [Sean]