This is part of the generic module

Dump the derivatives with respect to the input parameters for one or more objects (generally CVs, functions or biases).

For a CV this line in input instructs plumed to print the derivative of the CV with respect to the atom positions and the cell vectors (virial-like form). In contrast, for a function or bias the derivative with respect to the input "CVs" will be output. This command is most often used to test whether or not analytic derivatives have been implemented correctly. This can be done by outputting the derivatives calculated analytically and numerically. You can control the buffering of output using the FLUSH keyword.

Compulsory keywords
ARG the input for this action is the scalar output from one or more other actions. The particular scalars that you will use are referenced using the label of the action. If the label appears on its own then it is assumed that the Action calculates a single scalar value. The value of this scalar is thus used as the input to this new action. If * or *.* appears the scalars calculated by all the proceding actions in the input file are taken. Some actions have multi-component outputs and each component of the output has a specific label. For example a DISTANCE action labelled dist may have three componets x, y and z. To take just the x component you should use dist.x, if you wish to take all three components then use dist.*.More information on the referencing of Actions can be found in the section of the manual on the PLUMED Getting started. Scalar values can also be referenced using POSIX regular expressions as detailed in the section on Regular Expressions. To use this feature you you must compile PLUMED with the appropriate flag.
STRIDE ( default=1 ) the frequency with which the derivatives should be output
FILE the name of the file on which to output the derivatives
FMT ( default=%15.10f ) the format with which the derivatives should be output
RESTART allows per-action setting of restart (YES/NO/AUTO)
UPDATE_FROM Only update this action from this time

Only update this action until this time

The following input instructs plumed to write a file called deriv that contains both the analytical and numerical derivatives of the distance between atoms 1 and 2.
DUMPDERIVATIVES ARG=distance,distanceN STRIDE=1 FILE=deriv

(See also DISTANCE)