Serious Eats - seriouseats.com

There is a difference between the smash the raw meatball on the grill to ensure maximum contact with the hot surface versus flipping over a burger that has already cooked on one side and then smashing it. The first method makes a nice crusty outside while the other way just makes for hockey pucks.

There is no best way of cooking burgers. Each method has its own merit and its a matter of prefference. Smashing burgers might seem to be against everything we know to achieve a fluffy and juicy burger, but who says that it has to feel like that all the time? At my restaurant I serve burgers using both methods and each method yields a different kind of burger. I love the smashed burgers for all classic diner style burgers, however I preffer gently hand formed patties for gourmet burgers (not going to be smashing kobe beef on the grill). Smashing burgers on the contrary is the best way to hide beef imperfections as it has a larger crisp surface area that tend to absorb most of the seasonings as well as cooking it more thouroughly. The quick searing by the smashing action might seal some juices, but those juices are trapped inside the patty and not in the meat particles. The extra juice is actually being picked up from the flattop. Try to go for the first burger and see how dry it would be in comparison with the 30th. I think that's one of the reasons why burgers taste better when it gets busy.