"Dark L" is "L" at the end of the word or after a vowel sound. Example: ball, pull.
"Light L" is "L" at the beginning or before a vowel sound. Example: light, love.
There are 4 explanations of how to pronounce "dark L":
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1st explanation: the tongue tip must contact the ridge right behind the upper teeth.
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2nd explanation: before the tongue tip must contact the ridge right behind the upper teeth, you must curl the tongue and make sound like /r/ before make the /l/ as in the 1st explanation.
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3rd explanation: just put the tongue between upper and lower teeth is enough to make the dark L
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4th explanation: the tongue tip must contact the ridge right behind the upper teeth & the back of the tongue raises up (a British teacher teaches that).
see this vid: https://www.youtube.com/watch?v=pejo6YC_BnM and https://www.youtube.com/watch?v=U4En7vG1wV4 for more info
So what is the correct way to make the "dark L"?
Best Answer
I watched the first Youtube video you referenced, done by Rachel, and I think several of the observations are good, but overall, it misses the point. The important difference between dark l and neutral l in English is retraction of the tongue body for the dark l. (I'm calling the non-dark l "neutral", because I don't think l in English is truly light -- just sometimes not dark.)
The position of the tongue tip doesn't matter. When the body of the tongue is pulled back, that may affect the position of the tongue tip, but that's just because the tongue tip and the tongue body are connected -- they're both part of the tongue. It's the retraction of the tongue body that makes the l dark, whether or not there is any difference in the position of the tip.
Rachel talks about the pronunciation of "real" and observes, correctly, that there is an extra vowel sound that intrudes between the [i] and the [l]. But that is not what makes the [l] dark. When the tongue body is pulled back for the dark l, the body of the tongue has to move from front to back, since it was pushed to the front to make the [i] vowel, and it has to be pulled back for the dark l. To get from one place to the other, the tongue body has to move through an intermediate central position, and this is what Rachel hears as an extra vowel sound between the [i] and [l]. This central glide is predictable if you have a correct idea about how dark l is articulated.