Mechanically this mod does several things. Firstly, when you first unite Japan as a Daimyo or Shogun, you do not become an Empire but merely a Kingdom, and thus gain the title of Taikun (origin of our word Tycoon), which was what the Shoguns began to call themselves when they began dealing with foreign powers during the 19th century, both to distinguish themselves from the Emperor but also to assert themselves as nonetheless the supreme leader of Japan.

However, once the Age of Absolutism begins, if you have united Japan, you may choose the decision (if you have 100 adm power, and 2 stab) to abolish the Shogunate. This will result in the Shogun being deposed and replaced by the Emperor “Meiji” Yamato, a min 6 admin monarch. (Meiji was actually not the real Emperor’s name, but his “posthumous” title; the names of Japanese Emperors are a bit of a complicated issue, since their birth, reigning, and posthumous names are often completely different, not to mention rarely used to address them publically, but “Meiji” ((meaning “enlightened rule”)) was the name that the Emperor himself designated for his rule when he took power; thus, in this case, the game ruler is not necessarily supposed to represent the historical personage per se, but whatever Emperor undertakes the restoration which will be known as “Meiji”.) Moreover, once you have abolished the Shogunate you immediately gain access to the Decision to "Establish the Empire of Japan" which, once taken, will immediately grant you a unique base Monarchy Reform called the “Japanese Empire” Fixed Dynasty, Fixed Empire Government with several benefits (similar to the Prussian monarchy without militarization), and you will be given an option to use a new set of National Ideas, significantly stronger ones, inspired by the benefits of the Meiji Restoration: the actual bonuses themselves based on combining the best ideas from several Clans, the daimyo, and the default Japan ideas. (see below for details)

However, abolishing the shogunate has some steep costs. Firstly, two fairly large noble rebellions immediately rise up in reaction, and, secondly, for the next 10 years Japan gets the modifier “Bakumatsu”, which increases global revolt by 10, stability increase cost by 25%, but decreases autonomy (monthly) and corruption (yearly) for the duration. If you manage to survive the next 10 years intact, the Empire of Japan will be much stronger than before, and poised to expand into the pacific and combat western powers.
(Note: I divided the "Abolish the Shogunate" and "Establish the Empire of Japan" into two separate decisions in order to ensure that the Yamato dynasty remains permanent.)

Here are the relevant Stats:

Japanese Empire Reform grants:

Fixed Rank Empire:
Max Absolutism +10
Yearly Legitimacy + 1
Monthly War Exhaustion -0.02

Features:
Fixed Dynasty (Yamato)
No Royal Marriages (Japanese Royalty never married outside of Japan)
Can no longer become Emperor of China.

Bakumatsu gives:
Global Unrest +10
Global Autonomy -0.05
Yearly Corruption -0.2
Stability Cost Modifier +25%

Imperial Japanese Ideas:
Traditions:
Idea Cost -10%
Domestic Trade Power +25%

Ambitions:
Discipline +5%

“Son of Heaven”:
"Our Emperor is not merely the recipient of, as the Chinese say, the Mandate of Heaven, but a direct descendant of the supreme goddess of the universe, Amaterasu. There is no greater divine sanction in this world."
Yearly Legitimacy + 1
Global Unrest -1

“Establish the Imperial Japanese Army”:
"For too long we have relied upon a small population of hereditary nobility for the backbone of our armies. With the rights of the Samurai curtailed, we must open the armed forces to all men, regardless of class, and implement a policy of nationwide military conscription."
Global Manpower Modifier +20%
Land Forcelimit Modifier +20%

“Adapt European Battle Tactics”:
"For the majority of our history nearly all wars have been internal and thus our armies are still fighting in the style of the blade and bow of centuries past despite the introduction of firearms from the west. We must study the battle tactics employed in Europe, and as rapidly as possible adapt them to our own troops if we hope to create a truly effective Imperial Army."
Infantry Power +10%
Fire Damage +10%

“Imperial Rescript on Education”:
" “Advance public good and promote common interests; always respect the Constitution and observe the laws; should emergency arise, offer yourselves courageously to the State; and thus guard and maintain the prosperity of Our Imperial Throne coeval with heaven and earth.” "
Global Institution Spread +10%
Technology Cost -5%

“Imperial Rescript to Soldiers and Sailors”:
"The Imperial Rescript to Soldiers and Sailors established the effective martial philosophy of the Japanese military for the rest of the Empire of Japan’s history, stressing above all else the importance a soldier’s absolute loyalty to the Emperor, establishing a rigorous code of conduct and honor based upon a syncretic mix of Confucian and Buddhist concepts, and extending the concept of Bushido to all men regardless of class."
Army Tradition Decay -1%

“Modernization of the Imperial Navy”:
"As an Island nation we will all but require a large and formidable navy if we hope not only to extend our territorial influence, but to fend off the growing power of the empires of the West."
Naval Forcelimit Modifier +25%
Sailor Maintenance Modifer -5%

“Hakko Ichiu”
"We have proven to be the most prosperous and advanced nation in Asia, if not the world, blessed by Heaven itself; and yet with every year the European powers encroach ever further into Asia, dividing between themselves the once great empires of China and the Pacific. Now that we have achieved inner perfection we must look outward, and unite all the peoples of Asia under one rule: ours."
Core Creation Cost -10%
Aggressive Expansion Impact -10%