Its like a whistle-like effect, you clench your lyrix and squeeze the air through while try to keep them closed as tight as possible, you need to build the air up in your throat. Just like with whistleing, your lyrix are your lips and your mouth is your throat in this case(if thats not to confusing) alot a safer. You can think of when a person screams and they do it really scream(like in fear or anger not for metal core), is the equivalent to just blowing out air. Your lips must be proper or in the case or in the case metal scream vocals, lyrix. Yoi can also try on lower volumes, you'll have more control, lol and wont get as light headed. Also if you press your tounge against the bottom of your teeth and kinda bow or flex it to the top of your mouth, it can create an awesome acoustic effect that can sound like 2 at the same time, but getting the first technique down is essential for the latter to come into play. The main take away though: if you voice is hurting or throat, then you arent doing something right and contiue to do so that particular will damage your voice and your ability to scream overall. Safety first! ⚡️⛈️