“Bumping” is a simple mechanism that advances the line in a string trimmer. You simply bump the head of the machine on the ground to feed more line out. This has been a standard mechanism in string trimmers for years, but newer models have been using other methods. Bumping too hard can damage your trimmer, and your trim won’t stay a uniform height. The advantage of a bump feed is that it’s simpler and more reliable than a button or auto-feed system. Jams are less likely to occur and easier to clear.