Electrically dimming a tungsten bulb will cause the color temperature to drop, making it progressively more orange. Scrims can be thought of as color-safe dimmers, since they cut the output without changing the color or noticeably diffusing it.

There are scrims that are designed for special purposes. A half scrim only covers half of the light's face, the other half being open. By rotating it in front of the light you can choose to cut the top, either side, or bottom half of the beam. Since the scrim is so close to the face of the light, the drop in intensity between halves actually happens less abruptly than you might imagine.

A graduated scrim has thicker layers of screen on one side that feather off to a thinner screen layer on the other side, and can also be rotated to place its effect where needed. This provides the light's full output on one side of the beam and progressively dims it on the other side, useful if lighting from an angle where the light beam is striking some things that are closer than others, and overexposing them.

You can stack two scrims together to multiply their effect. For larger sources, such as soft lights, scrims arent really practical, and so cutting the ouput without resorting to electric dimmers usually means adding neutral density gels to those.