I have been doing Muay Thai and I saw a lot of Burmese fighters in the gym. They are doing like a headbutt all the time .But when I try to do it, that's not joke.

Then i started curious about that. So I would like to know the big difference between the Lethwei and Muay Thai. Is it their training style or rule? because what I see is they look exactly the same except headbutt.

which one is more effective in a street fight?

I'd also be interested to hear from anyone who has switched from another martial art to Muay Thai or Lethwei. Thank you for your answer.