One of the three improvements for the Shovel Blade the Trench Blade causes dig piles to be dug up in one fell swoop. Though it is a time saver, digging up piles in one motion doesn't really augment Shovel Knight's abilities much, so it may be advisable to spend gold on other purchases. In a pinch, the Fishing Rod can make a good substitute, although at the cost of some magic.
Notably, digging the dig pile in front of the hedge farmer with the Trench Blade upgrade results in a different exchange.
When playing amiibo mode, the only way to use this upgrade is by equipping the special move in "Shovel Upgrades".