if you right-click on the word ‘Position’ in the align to spline tag and choose Animation/Show F-Curve it will bring up the Timeline in F-Curve mode – the line you see is the motion of the Position parameter (which controls the wheel’s movement). If the line was perfectly straight (while increasing upwards) it would mean the speed is constant. The curve by default does an ‘ease-in/ease-out’ it starts in slowly moves at a fairly constant speed then stops slowly, this makes for more natural looking motion (usually). You can change the rate of change by moving the handles of the curve (use the Move Tool). Nearly flat means slow speed change, nearly vertical is the most speed possible.
By default both points are selected and if you move one it will ‘mirror-move’ the other, if you want to adjust only one end of the curve make sure only that point is selected.
I’m guessing you want the wheel to move in and brake quickly, that would mean shortening the handle but leaving it still the same direction. If you wanted an instant stop that would mean align the handle with the spline so there is no slow down. Play around with it (particularly with a shorter timeline so it’s quicker practice).