A pedal slide is produced by playing a string and then sliding the relevant pedal from one position to another. The string is plucked only once.
Gut string pedal slide.
A slow wire string pedal slide.
A pedal slide sounds somewhat similar to a glissando between two semitones. Nevertheless it is important to remember that the pitch does not change gradually from one to the other. The legato character of the slide and the absence of a second attack makes the slide sound like a glissando although the change of pitch actually happens stepwise.
Pedal slides are indicated with a short line between the two semitones. The line tilts in the direction of the pedal change. The pedal change must also be notated. Always notate enharmonically in context with the pedal movement.
A pedal slide will always be loudest when the slide is done just after the string has been struck. In the lower register the strings have a long vibration time, therefore a slide will be audible even if the string has already been sounding for quite a while. In the high register however, the slide will only be effective if it is performed immediately after plucking the string. The pedal slide itself can not be stretched over time since, as said above, the pedal does not change the pitch of the string gradually.