Alma 4:16 - 4:20
Alma 4:16 And he selected a wise man who was among the elders of the church, and gave him power according to the voice of the people, that he might have power to enact laws according to the laws which had been given, and to put them in force according to the wickedness and the crimes of the people.
Alma is now wants to devote his time to working with the people to get them back to living the gospel as they should. To do this he wants to gives up his position as chief judge. To replace him as chief judge, Alma selects one of the elders of the church. Alma was looking for someone he knew would enforce the laws on the books regardless of the wickedness of the people.
I assume that he is talking about that this person would not be afraid to enforce the law with greater effort if the people were becoming even more wicked and would not back off. Not only that but that he would not slacken the laws if the people were to pressure him to do so.
Alma 4:17 Now this man’s name was Nephihah, and he was appointed chief judge; and he sat in the judgment-seat to judge and to govern the people.
The person Alma chose was named Nephihah. He was appointed the chief judge and took over the duties of that very important position.
Alma 4:18 Now Alma did not grant unto him the office of being high priest over the church, but he retained the office of high priest unto himself; but he delivered the judgment-seat unto Nephihah.
Nephihah was not made the leader of the church as well. Alma retained that position.
Alma 4:19 And this he did that he himself might go forth among his people, or among the people of Nephi, that he might preach the word of God unto them, to stir them up in remembrance of their duty, and that he might pull down, by the word of God, all the pride and craftiness and all the contentions which were among his people, seeing no way that he might reclaim them save it were in bearing down in pure testimony against them.
Alma had now come to the conclusion that it would take his full time efforts to try to get the people to come around again to living gospel principles. He needed to remind them over and over again of their obligation to their fellow man and try to wake them up to the fact that they were not taking care of the poor and those in need. He wanted them to see that the fighting for influence in the society was a waste of time and that they needed to realize they had a duty to see to the welfare of their neighbors around them. Alma could see no other way to do this other than bearing constant and continuous testimony to them in person.
Alma 4:20 And thus in the commencement of the ninth year of the reign of the judges over the people of Nephi, Alma delivered up the judgment-seat to Nephihah, and confined himself wholly to the high priesthood of the holy order of God, to the testimony of the word, according to the spirit of revelation and prophecy.
So in the ninth year, Alma quits being the chief judge and now confines himself to church service. He would use his testimony of the Jesus Christ and tell them of their fate if they did not lived up to the gospel as they knew in their hearts they should.