It is called Bomb(폭탄) Drink(주) = Bomb drink, 폭탄주. There are generally two ways to do this.
1. Soju + Beer
2. Western liquor (usually whiskey) + Beer
More after the jump.
There are many, many ways to drop the shotglass into the beer glass. You could simply drop the shotglass as if you were taking a Jager bomb, or...
Or you could do something baller like that.
Whiskey + beer is simple. Pour a shot of whiskey, and drop it into a half-filled cup of beer. This is a pretty strong drink, so it's bound to mess you up. Generally this is more popular among older people than young people.
Soju + beer has been gaining more popularity lately. They call it SOju+Macjoo(beer in Korean) and call it SoMac (소맥). However, SoMac could mean a mixed drink of soju and beer, so let's call it Soju Bomb.
|Those are shotglasses on top of beer.|
Now... if you're taking soju bombs, then just go ahead and do it. It's just a shot of soju plus half a glass of beer, what more could there be that I need to know? Proportions.
|Available at Korean online shopping malls, apparently.|
The difference besides the obvious different contents of alcohol in the drink, is the taste. If you add too little, you're just gonna be drinking the beer. If you add too much, you'll feel like you're chugging down soju, which ain't really pleasant. So if you get the chance, try the 3:7 ratio.
So why drink Soju Bombs instead of Sake bombs? Soju and Sake are pretty similar, but they definitely differ in taste and alcohol content. And nowdays this is one of the most popular alcoholic drinks in Korea... so you might as well give it a try if you have a chance!
Lastly, drinking in Korea is a big part of the culture. So enjoy it and learn it. But this in no way means Koreans are alcoholics and reckless drinkers, and justifies you drinking like an idiot. Have fun and enjoy, but know your limits!