Explain the Johnson Counters?

The Johnson counters are a variation of standard ring counters with the inverted output of the last stage fed back to the input of the first stage.  They are as well known as twisted ring counters.  An n-stage The Johnson counter yields a count sequence of length 2n thus it may be considered to be a mod-2n counter. The circuit above illustrates a 4-bit Johnson counter.

The state sequence for a counter is given in the table also as the animation on the left. The apparent drawback of this counter is that the maximum available states aren't fully utilized. The Only eight of the sixteen states are being used.