Why Kyle Chalmers' world record relay swim in Paris doesn't count

It's going to take a huge effort to stop Kyle Chalmers from taking home the gold in the 100m freestyle in Paris.

"King Kyle", as he is commonly referred to, showcased his world record pace in the fourth leg of the men's 4x100m freestyle relay to accomplish a thriller comeback and claim gold for Australia.

But although the 46.59 split comes in 0.21 of a second faster than Pan Zhanle's world record, Chalmers' time will not count.

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World record splits have been recorded on multiple occasions in relay events that have not been counted, although that didn't include Australia's Michael Klim back in 2000.

But why?

According to FINA rules, athletes competing in the lead leg of a relay are eligible to record an official time - or world record in this case - at a sanctioned meet, provided it is not in a mixed relay event.

Although Chalmers achieved the record in the men's relay event, FINA's ruling also has different provisions and restrictions around diving styles.

In individual events, swimmers are also required to remain stationary on the blocks when starting to identify potential disqualification for movement, a requirement that is not regarded in three of four relay legs.

The opening leg adheres to the ruling of individual races, where false starts for movement on the block post "take your marks" call are made, thus preventing advantages through momentum.

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Whereas, athletes in the second to fourth legs move on the blocks with only one part of their body required to be touching the blocks.

The relay dive start is faster than a regular diving start as athletes can anticipate the moment another swimmer touches the wall.

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Official times can also be recorded in individual heat swims, a swim-off, a split time recorded at the start of an event with a longer event - provided that the swimmer completes the event - or a time trial, record attempt event that is not achieved after a leading relay swimmer.

In the event, the lead swimmer achieves a world record by FINA provisions but the team is disqualified, the result will not be nullified given the disqualification was for a distance following their leg.

For Chalmers, the time does not count but it has sent a fierce warning to reigning Olympic champion Zhanle ahead of Wednesday's 100m freestyle final.