What holds the spinning top upright is not some force that acts against gravity. Rather, it is a strange deflecting force— the force that stays always perpendicular to the direction of the motion of the axis of the top. This diverting force [subverts] the instability: the top begins to fall, but then veers off, thus moving as shown in Figure 9.3. In the next paragraph I will give a feel of how this strange [gyroscopic force] comes about from Newton's second law.
A gravity-defying bike wheel. Let's use the bike wheel as our spinning top. Hang the bike wheel on two strings as shown in Figure 9.1, and spin it rapidly. Now, cut one string. Amazingly, the unsupported end of the axle will not drop down.1 Instead, it will start a slow precession. The faster the wheel's spin, the slower the precession will be.
Question. What makes the wheel stay up, defying gravity?
1 Provided you spun the wheel fast enough.