the label tip has some standard amount of characters, so a lengthly text gets truncated. Your flag is to override that default behaviour.
"If a partially hidden label
in any list view mode lacks tooltip text, the list-view control will unfold the label. If this style is not set, the list-view control will unfold partly hidden labels only for the large icon mode." It means that a label already must exist, and only the full display will enforced.
to read the fineprint.