Fifty-five years after leaving Jerusalem, Nephi gave the small plates to his brother, Jacob. There is no way to know how long before Nephi died that he gave them to Jacob, but we may surmise that Nephi felt that his end was near and that was the impetus to give up the plates.
When Jacob received them, they came with instructions. Nephi had created two sets of plates. The one that dealt with the reigns of the kings stayed with the kings. In this separate set of plates, which was to travel a very different line of transmission, the content was to be different. Nephi declared in 1 Nephi 9:9 that this set of plates was to contain the more part of the ministry. The instructions to Jacob continued the theme that they should only lightly touch history, but should rather deal with “preaching which was sacred, or revelation which was great.” It was all preaching or all revelation. It was the most important teachings and revelations.
That is the intent of the phrase “engraven the heads of them upon these plates.” In this case, the “heads” indicates the most important aspects.