When invoked Crop will display the A image and you must then select the upper left and lower right corners of the new, cropped image within the framework of the A image. Move the cursor box to the upper left corner of the region you want to preserve (you can use the row/column positions shown in the upper left of the screen to help). Once located, press the N key ('Next'). Then move the cursor box to the lower right corner position and press the N key again. JIMSAIP will create a new image in B that contains only the portions of the A image enclosed by the rectangular area defined by the two corners you selected. The image in A in unaffected.
Pressing the Q key before the last N key will abort the operation.