The ZigZag works as follows:

On the left part of the chart, it finds a pivot point with the NumderOfBars parameter, which is a bottom.

If the price action draws a top with the NumderOfBars parameter after this bottom, then the ZigZag draws a line from the bottom to the top.

If the price action makes a new deeper Low, without forming a top, then the ZigZag extends the descending line to the new bottom.

If the pivot point with the NumderOfBars parameter on the left side of the chart is a top, the algorithm is calculated inversely.