This only works on poured foundations. All of the work takes place inside.  The crack is filled with an epoxy or urethane foam sealing out water fast.  The expanding foam fills all the voids in the crack preventing water from seeping indoors.  If not repaired, it may cause further damage to your foundation, biological growth such as mold and mildew.  In most cases you can avoid costly basement waterproofing systems by use of an epoxy or urethane foam injection.