Another way to look at it: the game simulation runs in a certain number of frames per second. Each frame you send to the server the state of your input devices

Another way to look at it: the game simulation runs in a certain number of frames per second. Each frame you send to the server the state of your input devices (holding left arrow key, left mouse button is clicked, etc) and this is translated to moving, firing weapons, etc according to the rules of the game. You can't teleport hack by sending "left 9999" because your inputs only serve to tell the server what direction you want to move towards. Your maximum movement speed depends on the game rules and should not be under your control.