Wow. Some serious misinformation here, as usual.
#1 - Go talk to your accountant. Most people that play this game, do it without understanding the 'why' and the risks. Then they get burned.
#2 - Typically, in most states, those that play this game, enter 14 for their allowances in box #5 on the bottom of the form. (more than 14 allowances must be followed up with explanation documentation to the state and to the gov) They then put $0 (zero dollars) for additional amount withheld. This means NO TAXES ARE WITH-HELD AT EACH PAY PERIOD!! (Sound great? READ CAUTION BELOW!!!!!)
#3 - YOU ARE OBLIGATED TO HAVE SUBMITTED 80% of your ACTUAL TAX OBLIGATION by December 31st, of THAT tax year. IE, if you're gonna hypothetically owe $10k in taxes on some gross adjusted income, the IRS MUST HAVE RECEIVED $8k of it already come Jan 1 of the new year!! If they have not, THIS is where you get slapped with the penalty! You do not have to FILE your taxes; just send your tax payments IN. Your check MUST be accompanied by a tax payment form.
#4 - The remainder of your tax obligation is then due by April 15th as normal.
Unless you are VERY methodical about the extra cash that comes from your checks, you can VERY VERY easily get burned playing this game. Few people that try this really make it work; MOST people that try, fail to realize JUST HOW MUCH THEY PAY IN TAXES throughout the course of the year. When it comes down to Turkey Day, and they start doing the math, the 'holy sh!t' factor kicks in, and they realize they're in big trouble because they shaved 'just a little here and there', spending bits of that supposed tax deferred investment instead of properly investing it.
Good luck, Let me know if you need more info.
(FYI, I played this game for 3 years, never lost a dime, but never really came out THAT much further ahead to make it worth the ridiculous headache; because of the volatility of investment vehicles that would make this game worthwhile, it really only works in a STRONG market/economy where funds and short term investments payout well)
ETA - I will add, this was the case 3 years ago when I still played the game. The percentage obligation on 12/31 may have changed. The accepted threshold for undocumented allowances may have changed. (you may be limited to 10, or 12, or some other unknown number) As I stated in #1, TALK TO YOUR ACCOUNTANT before you even conceive of this investment strategy; lest you find yourself filing for an application for extension with the IRS and a setting up a payment plan the following year where you end up paying for TWO years taxes during the year. (yes, I've seen people that failed at this game end up having to do just that)
E.