Songs can be more complex than you think, they can have layers on top of layers of meanings, subliminal messages or tricks to make them more appealing, and to help the artist make the most out of the 3-4 minutes a song lasts, also it helps them share their message a little better. Allow me to explain. I tried to pick the most obvious ones so it's easier for you to understand. Also when you are listening to the songs, it's easier to notice the subtle details when listening with headphones, and on lower volume. You might also want to listen the songs when your mind is free, with no distraction, no reading the lyrics in the same time as listening or browsing facebook.
By mental image I mean that you have to use your imagination and your mind's eye to get the song completely. The artist paints a picture and you can look at it and perceive it however you want. While listening to these songs it's like watching a movie, you can also add sounds to the mental images. Try to listen to the songs and relax, each WORD can take you somewhere.
Examples:
Amon Amarth - Asator - This is one of my favorites, they even tell you to see and hear the image
"SEE the lightning strike"
"HEAR the thunder chariot ride"
Amorphis - The Bee - Another good example, this one is more complex
The song starts by making you focus on the space and the sky, notice how they say the words sky,stars,way,space in a different tone of voice, this is done to make you focus on those words specifically, then they say "suns are lit", followed by "stars are dimmed", notice how these phrases are being told again in a different tone of voice. I would explain the whole song but it would take too long, start by noticing these details first.
Nightwish - Elan is full of surprises, but I hope I explained enough to get you started. Now you might remember some songs from your collection that do just the same thing.
Hypnosis is very difficult to explain, there are entire books you can read to find out exactly how hypnosis works, but I'm going to try to give you a couple of examples.
In the song Are you ready by Disturbed they are using the word NOW about 5 times, they do it to make you focus and remember their suggestion, in this song the word NOW is always preceded or followed with a hypnotic suggestion. Repetition is a good way to make something stick with you, thats why most mainstream songs have only 3-4 words.
As I said before, they also change the tone of their voice when pronouncing certain words, but this is only one way to make you pay more attention. If you listen with your headphones, you will see that sometimes they say something in your left ear, sometimes in your right, sometimes in both, and sometimes they double their voices.
Also listen closely what is being said at the very start of the song and at the very end, there are studies that show we tend to remember stuff from the begining and end of things, but tend to blurr out whatever happened in between.
Some band also use fear to make people remember, because whenever your fear pathways are activated, you tend to be more carefull and receptive. I lost count of the band making you imagine falling or drowning, you probably know one song too, or maybe you just ignored that part. In any case, drowning and falling are the most common fears.
Also hypnosis can be done in countless ways, for example in the music videos of bands, not all of them of course. But they may contain subliminal messages. A very good example of a videos with lots of subliminat messages and use of hypnosis is
DEADTIDE - Begin the dream. Hypnotic suggestions in this song: SLEEP, DEEP, DREAM, FALLING, DOWN.
I hope my answer was satisfactory, if you have more questions about this, I will be happy to answer them.
Thank you for the suggestions.