Olympic/WR Swimming Question

In a lot of events you see the top swimmer/swimmers start out ‘under world record pace’. What does that mean? If the WR for 400m is 3:40, and the swimmer starts the first 100m on :54, is he under world record pace, or do they have the splits from the previous WR (like :52, :58, :56, :54 or something?)

Splits from the current WR.

Not all meaningful for IM events since each swimmer has different strengths.

They keep track of all of the splits during the race by 50 m or 100 m, and basically whoever is racing, if they’re splits are less than the WR pace that was set in the WR race, they on pace to break the WR (unless they die which is often the case)
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Thanks, that’s what I thought was the case.

edit: thanks liz too