{decrement $counter} takes one off the variable and prints nothing. A variable that does not exist yet, or holds an empty text, starts at -1. The variable is a global: the new value outlives the loop or block the tag stands in, so it can count down the condition of an {until} or {while} loop.

It is the short form of {set $counter = $counter - 1} and the mirror image of {increment}. The variable is taken by name - $counter, the $ written - and is never evaluated as an expression.

A value that is no number is an error naming the variable, and so is counting an engine variable such as $padPage.