The combatting principle of kendo is

Jousting.

In a jousting the players only go forward, never go backward during the match.

Kendo is like "repeated jousting with constantly re-positioning the straight line that two players are facing to each other".

The key point is that you can go backwards *before* your cutting movement starts, but once you decided to cut, the options are: 1. go through, or 2. smash into your opponent (tai-atari) in order to quash the opponent's cutting distance.

The only exception is when you start from tuba-zerial then do hikiwaza, but you must start from the correct distance which is tsuba-zeriai: the guard of both players touching to each other.

In any other cuts, there is no stepping backward, only go forward.