Tire plugs are made of short strips of leather covered with a gooey, unvulcanized rubber compound. When forced into a nail hole, the plug fills the hole and the rubber goo vulcanizes under the heat of driving to fully seal the repair.
When the hole is near the sidewall - many experts recommend a patch for when a tire puncture is near the sidewall, as the plug may not be able to completely seal the damage.
When the puncture is not straight - a diagonal puncture is more likely to need a patch solution.
Other Concerns About Plugs
Lots of experts also caution that a plug should not be applied without a patch unless a shop takes off the wheel for a visual inspection. Thorough inspection of the punctured tire helps catch additional damage that may go unnoticed when the tech is simply inserting a plug from the outside of the tire. The best and most effective solution, according to safety experts, is to combine a plug with a patch, to make sure the fix will stabilize the puncture.