Oscilloscope

The automotive scopes have some sort of adjustable delay gate that is displayed on the scope, adjustable by knob. You have one lead that hooked (in the old days) to coil neg/ points. When you move the delay gate "in time" from cylinder to cylinder, it clamps the coil neg to ground at that time, preventing that cylinder from firing. Same thing as going around the cap and shorting out one plug at a time