Here is an operational answer for the checker at a nuke plant that I worked at:
Verbatim compliance is required when performing a procedure like a quarterly test of an ECCS. Because operators have accidently turned the wrong switch or opened the wrong valve, they have been given the option to request a "checker".
And of course, due to the cost of having two licensed operator perform the procedure that normally would take only one person, the term "no knowledge" is a slight embellishment for someone who may not be fully qualified to perform the procedural task but can fill the position of "checker".
The role of the "checker" is to follow along with the procedure and confirm that the operator has put his finger on the correct switch (or valve) and identified the proper action to be taken (on, off, open, close, etc..). Once the checker has provided concurrence, the operator then procedes with the action. Sometimes the checker will initial the procedure along with the operator but usually only if qualified to perform the procedure in the first place.
This has reduced the number of mistakes and also gives the operator a fall back position, if a mistake was made but no checker was provided, when requested.