This technique involves using a filter (or in this case, filters) to slow exposure times enough to blur motion while leaving non-moving objects sharp. In brightly lit situations you cannot have shutter speeds in the 20-40 second range without way overexposing the image. But if you can “drip feed” the incoming light to the sensor by shooting through very dark glass, you can slow the shutter speed to 30 seconds or more. This can make waves or other water movement look silky. It can also make crowded areas seem empty of people or vehicles as long as those things are moving fast enough to not register on the sensor. Here are some examples of Black Glass photography on ocean waves. These were taken during the sunrise and sunset shoots from yesterday. I used three stacked filters with a total of 11-stops of darkening. Each stop reduces the incoming light by half, so these 30 second shots would have been taken at 1/60th of a second without the filters. At that speed, the water would have shown all the detail of the waves and current instead of looking silky.