Directive Studios Blogs
By greatly extending exposure times the Big Stopper has the effect of allowing anything that is moving in your image to become blurred or ghost like, for example clouds, waterfalls, rivers, and the sea.
The filter can also be used on cityscapes to blur people or on roads and motorways to blur traffic.
This filter requires the Lee Filter System/
Manufactures Site: http://www.leefilters.com/index.php/camera/bigstopper
Here's a few samples images from Lee: