Using a game in a way that its designers didn't intend (or didn't otherwise include in its rules) is itself a form of game design. The bigger the stretch, the more design work is being taken on by the group.
Stretch is usually created by hacking or converting a game, although significant house rules may also create stretch. A small tweak using the game's existing design language may not be a stretch, but introducing new design language or using it in a new way will stretch the system.
Views on stretch in role-playing games vary from highly negative (those who believe games should be used for their intended purpose, and that different games may be better for groups that want a different experience) to highly positive (those who want to test the boundaries of a system, or want to use their preferred system for every game they play).