|“||Perform two consecutive shovel drops to unlock a powerful charge slash!||”|
Dynamo Mail is a silver armor that costs 6000 gold to purchase. As the description states, after two shovel drops Shovel Knight gains a charge slash similar to the one the Charge Handle enables but without the movement penalties. After a certain amount of time without using the charge slash or performing more shovel drops, the charge runs out and more shovel drops must be performed to gain a new charge slash.
The other thing to note is that the Shovel drops don't have to hurt an enemy or destroy blocks, so bouncing off of Polar Knight's shovel or Baz's whip still creates charge. Also each sand block passed through counts as a shovel drop even though Shovel Knight does not bounce.