A pedal knock is a percussive effect produced with the pedals.
The pedal mechanism has a powerful spring that forces the pedals upwards. If a pedal is released from its slot suddenly, it knocks against the wood of the pedal slot and creates a pedal knock. This effect should be used sparingly, since it is not very friendly to the mechanics of the instrument.
The sound of a pedal knock is a wooden percussive sound combined with the of the pitch of the strings that the pedal, used to produce the knock, controls. If, for example, a pedal knock is performed by knocking the C-pedal from the C-natural to the C-flat position, the sound of the tuning discs hitting all the C-flat strings will be heard in combination with the knock.
A verbal explanation is necessary. The rhythm of the pedal knock should be notated with note beams below the two staves. Pedal indications replace the noteheads.
All the pedals can be knocked upwards. That is from the sharp position to the natural position or from the natural position to the flat position.