By default, all Brid outstream units have this feature disabled.
If you leave this option un-checked, the Brid outstream unit will start your ad waterfall as soon as possible. This usually happens on page load. Because Brid's outstream unit will start your ad tag waterfall as soon as it can, your ad content which should display in the outstream unit will get prepared as soon as possible and once your site user navigates to the place where the outstream unit should appear, hopefully your ad will already be prepared and ready to go.
As the Brid outstream unit is hidden from user view (which is the expected initial state of an outstream unit), the Brid player must pause your ad content in the background hidden from view until a viewer navigates to the part of the site where the unit player should appear. This means that ad providers may interpret this as non viewable ad content (even though that your ad content hasn't really started playback yet).
If you find that viewability is a concern for your ad inventory then you may want check this option enabled. If you enabled this option, the Brid outstream unit will start your ad waterfall as soon as it can but much later then if left un-checked. If this options is enabled, your ad waterfall will start as soon as a site user navigates to the place where the outstream unit should appear. Depending on the size of your ad tags waterfall it may take some time untill you can see your ad display in the unit.
If you find that your ad tags are having issues with viewability statistics, consider enabling this option.