I've looked in a lot of websites and forums but no one really explains well how does they work in some points:

1. When i level up a perk to level 100, then i make it leggendary (and the Skyrim's logo pops down) the resseted, now leggendary perk, is more powerful? Now that the perk is leggendary it will give me more damage or bonuses?

2. Many people says, that when i reset a level 100 perk, the returning points that it will give me, i will be able to store them ONLY in the previously reseted perk?

Let's suppose, i want to make blacksmith leggendary because i want my points back to put them into One-Handed Weapons, can i do that? Or the points will be able to store only in the blacksmith perk?

1- All it does is reset your Smithing back to 15. No extra bonuses.

2- Alright, so you make Smithing Legendary. With those points given back, you can choose whatever skill you want to invest in. If want to put in some points in One Handed, you're free to do so.

Okey thank you so much!! at least i know the truth nao!

It's not a bad idea to choose a skill that you use frequntly. After it's reset to zero, you can begin leveling up and by the time you get it back to 100, you'll have gained about three character levels. If you have the patience, there's no real limit to how high you get your character. Very tedious and rather pointless though.

I'm 128 in Destruction right now (thanks, Uncapper), and would NEVER consider making that skill Legendary.

Something to note is that it is generally a very bad idea to make Legendary your main combat skill, such as one-handed or archery, as once you do, you pretty much lose all forms of dealing damage, on account of the drastic drop in how hard anything hits.

...that was a very long sentence...

yea well, getting the main attack perk to reset it's quite hard to do, or it needs a lot of time to restore back,

If you use the Warrior Stone, you'll get one-handed or archery up above 50 with half a dozen kills.

And they don't have to be major opponents. Goats, mudcrabs, wolves...they all work. It's very easy to exploit.

The only payback you get though, is higher magicka, health and stamina.

And the opportunity to take more perks. This can be useful if you're focusing on only a small number of skills, since you may stop leveling up before you can take all the perks you want.

That's true. I was thinking past level 81 with all skills at 100. This exploit doesn't require topping off all of them though. Hell, if you're persistent and have a lot of spare time, you can do it with one.

Just to make sure I wasn't dreaming, I consoled my one handed to 100, made it legendary and went hunting. First, a mudcrab sent it from 15 to 25. Another mudcrab made it 31. A fox to 35. Two more mudcrabs to 38 and 41. Then boredom set in.

Once you get to about level 50, you increase one level per kill until level 90. Then it becomes one lever per two kills until 100.

How is it leveling so fast? It doesn't level that fast from the start, not even with a daedric weapon.

Did you make use of the Warrior Standing Stone just west of Riverwood? If you want to boost mage or stealth skills, there's a stone for them too, although all of the related skills don't boost at the same rate. For instance, Stealth Stone enhances marksmanship and that will increase just like one-handed. But lock picking, sneak and pickpocket go much slower.