The "one kilo
rule" refers to the International Weightlifting Federation
rule change in weightlifting
competitions in 2005
which allowed lifters to call for smaller weight increases after successful attempts.
Prior to the one kilo rule, weight increments were made in multiples of 2.5 kilos, and so essentially these were the multiples that coaches and athletes had to think in when they were trying to figure out what they needed to lift to take the lead. For example, if someone was shooting for at least a spot in the top 3 positions (getting a medal), and they were trailing behind the number three lifter by 5 kilos in the snatch, they would need to jerk at least 7.5 kilos more than the number three position to take his spot (so he would total 2.5k higher), assuming everyone's of equal bodyweight (in the event of equal totals, the athlete with the lowest bodyweight wins). Pretty simple once you get used to thinking in these terms, because really you're just thinking in terms of fourths, or quarters.
The advent of the one kilo rule allowed lifters to call for weight increments of a minimum of one kilo. This created a much more tactical environment in competition, because consistent lifters could essentially just conceptualize their next attempts as being the same weight, since the weight increase is almost negligible. But if you were behind, and you wanted a shot at medaling, you'd have to take bigger jumps, which meant you would conceptualize your next attempt as being "heavier," even if by only five or seven kilos, because no matter what, you would feel the difference.
Andrew Charniga, a very well-researched expert in the science of weightlifting (a lot of his work is based on scientific studies of athletic biomechanics, establishing motor patterns, correlation between attempt success rates in competition and particular training or competition habits, etc.), notes that if an athlete approaches a weight knowing that it will "feel" heavier than a previous successful attempt, there's a high probability that the athlete will deviate from the desired motor pattern in an effort to pull harder or get under faster. And maybe those efforts will actually be productive and result in a successful lift, but a lot of the time, it just means that the athlete does not adhere to good positions or timing, and it results in a missed lift.
The idea is, first of all, be consistent, and then use that consistency to force everyone behind you into taking a psychologically-challenging jump to a higher weight, while all you have to do is coast along in one or two kilo jumps and hopefully just keep repeating the same effort.
Competitions have become much tighter and closer because of this, because the key is suddenly consistency. If you want to take the lead, open with a fairly heavy weight, and then maintain that lead with 1 or 2 kilo jumps. And if you are not so consistent but you're strong enough to challenge the guys who are, get on the board first, then start swinging for the fucking fences and hope you can catch up in the clean-and-jerk if you end up missing an attempt with the snatch.
This benefits talented guys like Taner Sagir, who is easily one of the most consistent lifters in the world when he's not injured. At the 2006 Santo Domingo World Weightlifting Championships, he opened with 162k, then proceeded with 164k and 166k. Li Hongli, notoriously strong but also a little inconsistent in the clean-and-jerk, opened with 163k and finished with 167k after missing his final attempt at 170k. Sagir's a kilo behind, but he knows exactly what he needs to do: jerk two kilos more than Hongli. The Chinese monster (Hongli is one of the most muscular lifters in the 77k weight class, probably second only to Russian Oleg Perepetchenov), opens with 192k and is successful, albeit a little shakily. Sagir comes out with 195k, misses on his first attempt, but comes back to take it on his second with authority. What he has done is forced Hongli to take at least a four kilo jump: Sagir is more than capable of just applying the same technique for 196k and 197k, so Hongli needs to re-establish at least a two- or three-kilo lead in the clean-and-jerk to force Sagir to go for 199k or 200k in order to keep the number one spot. All eyes are on Hongli when he goes for 197k, but he misses it on his second and third attempts, and Sagir walks away with the gold medal.
But if someone's gunning for big numbers no matter what, then you've still got the balls-to-the-fucking wall attempts that really make this sport the beautiful thing it is: like at the 2007 Junior Worlds, where Ilya Ilin ensured his gold medal position in the 94k class with 215k and then 217k in the clean-and-jerk, then came out hard with 225k and nailed it just to show he's world record material.
http://www.iwf.net/iwf/doc/technical.pdf - IWF rulebook
http://www.sportivnypress.com/documents/79.html - Charniga's commentary on the tactical evolution of weightlifting competition