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On a rotary pot, clockwise is maximum sharpness or peaking, all the way ON.
Here are a couple of pages to explain it. Figure 13-127 shows an ideal "aperture compensation" circuit of the type used in studio cameras and not in receivers due to the need for delay lines. The idea is to increase the apparent sharpness of edges and the contrast of small details by adding a pre-shoot and a post-shoot to every vertical edge. Thsi is horizontal peaking, which can be done with delay lines or RLC circuits. Really expensive circuits in studio cameras also would have vertical peaking (aperture correction) for horizontal edges. The really expensive plumbicon studio cameras even had different aperture correction for diagonal edges.
Getting back to the figures: Most TV sets had a sharpness control consisting of a RC circuit with a variable R in series with a C, connected to the cathode (or emitter in a transistor set) of the video output. This circuit can vary the amount of high frequency gain, but it only affects the smear or overshoot following the edge (that is, to the right of the edge). However, the phase response of the video IF amplifier could produce a permanent pre-shoot (to the left of the edge). So, it ws common to design the IF to produce some medium amount of preshoot, and tehn use the sharpness control to vary the overshoot. This is not ideal, but could compensate for viewer taste, antenna tilts, etc. to some degree.
If you look at the frequency response (left side of Fig 13-127, you see that there is a steep slope on the high frequency cutoff when the peaking is at max. If this slope becomes too steep, the transient response will not only have a pre-shoot and overshoot, but the waveform will ring, which will look like two or more preshoots and two or more overshoots. To the eye, these appear as extra edges or ghosts, which is what you see when you turn the sharpness control clockwise, especially if you also have the fine tuning adjusted to emphasize high frequencies in the picture.
Now take a look at Fig 13-129. This circuit was invented in a current-mode design by my (now deceased) boss Howard Jirka at Zenith. I modified it into the voltage-sensing form you see here because it used fewer IC pins that way. It has the advantage of being the simplest and cheapest possible approximation to the ideal delay line circuit, and produces both preshoots and overshoots. My colleagues at Zenith and I combined this into an analog IC that did the peaking, contrast control, and edge gating all in one circuit. Edge gating was done by sensing the first derivative that appears across the R in the RLC. By gating, peaking only worked on edges and did not enhance noise in flat areas. I don't recall for sure, but I think some sets had a customer control to turn the edge gate on or off. If the signal wasn't weak, you could disable the gating, and the peaking effect would work on texture in flat areas also.
The nominal setting of peaking (at midrange on the control) was a matter of taste. Different brands of sets had different degrees of it, and it varied over the years at each manufacturer as different personnel got to decide what they liked and thought would impress potential buyers on the sales floor. All sets had more than a moderate amount, such that black letters always had exaggerated white edges - but this made the pictures look really "sharp" (with high contrast for small details) when viewed on the sales floor. In sets with the older overshoot-only adjustment, the customer had to live with whatever preshoot had been designed into the IF response.
The amount of peaking needed for a sharp looking picture is reduced if you have more real definition, as in a high defintion TV picture, but designers still put some peaking in, even in HDTV. Videophiles generally complain that the factory setting is too high and reduce it or set it to minimum.
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