Unlike a moving average, the tenkan-sen uses only the extremes of the window, so it goes completely flat whenever the high and low of the last nine bars are unchanged. Those flat stretches are themselves informative: they mark balance.
It is the fastest line in ichimoku-kinko-hyo and is used as a short-term trend gauge and as a dynamic level in fast moves. Its crossing of the kijun-sen is the system's basic momentum signal.
Because it ignores everything between the extremes, a single spike moves it sharply and then it sticks. That behaviour is different from a moving average and catches out traders who assume it behaves like one.
Related: ichimoku-kinko-hyo, kijun-sen, kumo-cloud, moving-average, senkou-span