The common theme with all of these resurrections is that they're based in esotericism and mysticism. Using modern sensibilities to try to make sense of the story won't usually work because modern science doesn't agree with it.
It is challenging to discuss concepts like near-death experiences because a person with modern sensibilities may try to explain a mystical element by discussing it scientifically. (Good luck trying to explain death!)
How I view Luffy's death is found below.
- Luffy was going to die because his life force energy was diminished. However, when his fruit awakened, it invigorated him with new and abundant life force energy. As a result, he stopped being upset and became happy. Being stressed out causes Haki to become difficult to use. When he became happy, his mind/body/spirit link had harmonized resulting in changes to his personality, his body, and his Haki.
The awakening was likely also based on Luffy experiencing a loss of earthly desire. The way he regards death tends to be like, "I lived the way I wanted to, so I'm going to die happy!" He smiles.
When he actually died.. he may have experienced a moment where he stopped caring about everything as much and became, "spiritually enlightened." This is in line with Buddhist beliefs that a person must overcome earthly desire before they can achieve a state of nirvana and stuff.
Oh yeah that's the word for it too... Luffy may have experienced Nirvana/Heaven in a sense and that may have been what changed him so much. Something along these lines.
The Drums of Liberation are just a result of Luffy's heart being harmonized with his new abundant Haki. His heart beats faster and in a rhythm because he's happy and he has adrenaline rushes. Think of it like ADHD Sing-Song Heart, which normally wouldn't help someone because it's actually high blood pressure as a result of hyperactivity, but Luffy's body is different because his body is mystically endowed with rubber characteristics. His heart won't explode from going fast because it can stretch.