There is a portion of your bullet that is called the ogive or where it contacts your lands. It is some where on the rounded portion of your bullet.This area is where you space back from the lands. It is just another way of making your rifle accurate. This has to be done very accurate, if you get to close it can build high chamber pressures. It takes some shooting to get it right for your rifle.The comment about it having no practical use is very true for most hunting but for benchrest shooting or long range it is a must.