When you choose the bin tool, the mouse pointer looks like a double-headed arrow between a pair of lines. Only the histogram supports the bin tool.
Clicking or dragging the bin tool shifts the location of the histogram bins. Clicking near the horizontal axis reduces the number of bins and makes the bars wider. Clicking near the top of the plot increases the number of bins and makes the bars narrower. Dragging the mouse pointer horizontally does not change the number of bins but changes the position at which the bins start.
When the pointer is at the left edge of the histogram, the bins start at an integral multiple of the bin width. When you move the pointer toward the right, the bins are offset by an amount that is proportional to the distance between the pointer and the left edge of the histogram.