what is 'premove'?

and why is there so much controversy regarding the subject lately? (I have noticed lots of angry posts)

Basically an option in live games where you can mouse-move out a legal move EVEN when it's still your opponent's turn, and once he makes his move, the server will automatically play your "canned" response  if it is still legal.

Why do people do it?

- Pre-move costs 0.1 seconds or even less on most servers, so blitz/bullet gamers tend to save on clock, especially in crunch times. After all, bullet/blitz games are usually all about the clock situation. 

- Canned opening responses like 1...d5 to anything etc. are possible 

- There's a certain intimidation/gamesmanship factor of facing pre-move after pre-move, where your opponent is mercilessly "forcing" you to think on your own time.

- Now for the dark side of pre-moving : If your opponent has seconds on the clock and so do you, playing nonsense (dummy checks, forcing piece sacs, etc.) actually works to your advantage as you burn his clock ... pre-moving these moves is actually icing on the cake!  

Depending on how much of a chess purist you are, this is either seen as "win at all costs" competitiveness or extreme douchebaggery.

But can't this screw up, i.e. the pre-move could be dead wrong if my opponent plays something I didn't expect that requires a different move?

Sure, I've seen players goof up often (and I have as well) but once you get the hang of it, you don't pre-move unless you are pretty sure the lines are forcing or are are trying to distract/confuse and eventually flag an opponent who's almost run out of time.

Controversial, yes ... because one of the common complaints of online blitz/bullet chess is lag ... laggy players *can* get an unfair advantage as many chess servers handle the lag strangely and readjust the clock in their favor. Club that together with pre-move and you often see insane situations where a laggy person with < 0.5 seconds ends up pre-moving nonsense moves and flagging a person with 10+ seconds ... you JUST won't see this happen in a real OTB blitz game.  I can see a lot of people have issues with this.

 Ok thanks for explaining that all for me.

Pre-move is legitimized cheating.  In face to face chess you can't touch your pieces until your opponent makes his move and hits his clock.  Pre-move is touching your pieces while your opponent is moving.  Just because all the chess sites have pre-move doesn't make it fair.  It just means that the chess sites have forced it upon us.  Players who think this is fair are the kind of people who think that just because an institution puts their weight behind a ridiculous feature that make it okay.  Pre-move is garbage, plain and simple.