I saw this same video last summer around when I started my bass traps. The salt trick works - I used it to keep some pine bits from sliding around while clamping so I could make feet for stands.
Salt not sand as sand would blunt any blade used on subsequent cuts I presume. I mainly tend to screw and glue joints so the screws do the alignment and haven't seen this trick before, but I suspect it will come in handy one day...