I see this; > 252+ Atk Technician Mega Scizor Bullet Punch vs. 252 HP / 0 Def Abomasnow: 306-360 (79.6 - 93.7%) -- guaranteed 2HKO.
How does it work?
I don't know what you mean. That is the equation shown on the Pokemon Damage Calculator. It basically says a Mega Scizor with 252 EVs in Attack using Bullet Punch on Abomasnow with 252 HP EVs and 0 Defense EVs deals 306-360 damage, which just happens to be 79.6%-93.7% of Abomasnow's HP, and will always be a 2HKO. To use the damage calculator, just enter the details of the attacking Pokemon and the defending Pokemon, select the move the attacker will use and it will come up with a formula similar to your one, and you can find out how much damage the attack will do.
I assume you're asking not how the damage calculator on showdown works, but the actual broken down formulas for the damage to be calculated. I'm not too good at explaining loads of math, but Serebii is. http://www.serebii.net/games/damage.shtml
If you were asking just about how he arrived at that answer, look in the Pokemon Showdown damage calculator. http://pokemonshowdown.com/damagecalc/