The deadline dates back to 1911, when the second Thursday in March was decreed to be the cut-off point for any buying and selling. It was later moved back to the fourth Thursday in March, which means that clubs have until this Thursday - 27 March - to strengthen their squads for the final push, or take the chance of letting someone go and boosting the bank balance. With a fifth of the season still left, these deals can mean everything or nothing.
You need a steady nerve in this crunch-has-come atmosphere. Who's worth the risk? Do we have the money? Go for a razzle-dazzle striker who might back-fire? Or play it safe and invest in solidity and experience? What's more important? Scoring goals or stopping them? Or maybe it's best not to rock the boat at all - stick with your existing squad, show your players you've faith in them, and hope they'll reward you.
It's a game of brinkmanship that few clubs down the years have managed to resist, although a refusal to be drawn into it can say as much about a team's feeling of self-worth as does a headlong rush into the market- place. Most supporters can bring to mind a player who left his entrance for the final act of the season, and either took the stage by storm or fluffed his lines completely.
Manchester City fans of a certain vintage still shudder at the memory of Malcolm Allison's signing of Rodney Marsh in March 1972 when the team were going for the old First Division title. It looked like the clinching buy. The magic of Marsh allied to the class of Lee, Summerbee and Bell. In fact it turned out to be one of Allison's great tactical blunders. Marsh never gelled with the others, and a stuttering City ended up in fourth place.
City, however, can also lay claim to one of the game's most inspired deadline signings. When Niall Quinn arrived from Arsenal in 1990, they were in dire trouble near the bottom of the old First Division. Quinn played nine games, scored in four of them, and every goal meant either a point or three for his new club. They were up to 14th by the end of the season.
There is a clear pattern to deadline buying. Whether you are desperate to stay out of trouble, or desperate to keep the promotion drive going, forwards are what you look to. Of the big-name March signings of recent years, the vast majority are men who are there to put the ball in the net. "The gambling instinct takes over," says Ray Wilkins, the former Queen's Park Rangers manager. "It's a very difficult time. You can understand managers not wanting to be too cautious."
Sometimes the priority for a defender is obvious. That's how Steve Perryman came to end his 17-year career at Tottenham Hotspur. It was late in the 1985-86 season. Oxford United had just been beaten 6-0 by Liverpool, and were seriously in the mire. Perryman arrived, and the team stayed up. More recent examples of defenders moving at deadline time are Graeme Le Saux (Chelsea to Blackburn on an exchange in 1993) and Darren Peacock (QPR to Newcastle for pounds 2.7m in 1994), although in the latter case the move was driven as much by QPR's need of the money as by Newcastle's desire for a top defender.
Selling at this time of the year can send out wrong signals just as buying can offer a boost. When Manchester City (the regularity with which they have figured in recent deadline activity is perhaps revealing) sold Garry Flitcroft to Blackburn late last season, the demoralising effect on the team was part of the reason why they went down.
When it comes to it, though, it's the forward brought in to get the goals over the closing stages who is really at the sharp end. Iain Dowie of West Ham, another club who like to do business at this time of the year, knows all about this. His first spell at West Ham began just before the deadline in 1991, when the club were going for promotion from the old First Division. Dowie was stepping down a division from Luton Town, but in many ways the challenge was much greater.
"It can be daunting," he says. "You know why they've bought you. It's down to you. But all of a sudden you've got new team-mates, a new system, a lot to adjust to, and there isn't much time." But when he scored on his debut there was no looking back, and West Ham went up.
By next Thursday, a new set of cavalry will be in place, men who probably won't want to think about the hope that has been invested in them. In a few short weeks it will be death or glory - or just plain relief. Wish them luck.
The Emergency Strikers XI
Frank McAvennie: Celtic to West Ham, 1989, pounds 1.25m. Impact: West Ham were bottom of old First Division. But they went down as their former star failed to score in five games.
Niall Quinn: Arsenal to Manchester City, 1990, pounds 800,000. Impact: City were 18th in the old First Division and facing relegation. Quinn's four goals in nine games helped lift them to 14th.
Iain Dowie: Luton to West Ham, 1991, pounds 480,000. Impact: Hammers were going for promotion from old Second Division, and four goals by Dowie in 12 games helped them do it.
John McGinlay: Bury to Millwall, 1991, pounds 80,000. Impact: At nearly 27, this was his big break, but though he scored in the play-offs, the then Second Division club missed promotion.
Robert Rosario: Norwich to Coventry, 1991, pounds 600,000. Impact: A lot to pay for someone Coventry then only had twice on the substitutes' bench over the closing stages. But they won the annual struggle to avoid the drop.
Clive Allen: Chelsea to West Ham, 1992, pounds 275,000. Impact: It was Mc- Avennie all over again. One goal in four matches could not prevent West Ham being relegated from old First Division.
Efan Ekoku: Bournemouth to Norwich, 1993, pounds 765,000. Impact: Inspired signing from lower divisions by Mike Walker, as Ekoku's three goals in four matches helped Norwich into Europe.
Peter Beagrie: Everton to Manchester City, 1994, pounds 1.1m. Impact: City were hovering above relegation zone, and Beagrie, an ever-present for the last nine games, helped keep them up.
Anders Limpar: Arsenal to Everton, 1994, pounds 1.6m. Impact: With sale of Beagrie, Everton went for another left-winger, and avoided relegation on the last day of the season.
Jan Age Fjortoft: Swindon to Middlesbrough, 1995, pounds 1.3m. Impact: Icing on the cake for First Division leaders. Fjortoft's three goals in eight games helped them go up as champions.
Ashley Ward: Norwich to Derby, 1996, pounds 1m. Impact: Second in the First, Derby felt they needed more up front. Ward scored only one in seven games, but promotion was secured.