I think, we can exclude any model that would require the entire line of Hastati to move back in the middle of hand-to-hand combat and the enemy patiently waiting until the Principes had proper alinged themselves.
One possiblity would be that the Hastati would be pushed back by the enemy and retreated through the gaps of the Principes. They would be anchored on this second line with the front echolons of the Principes acting as some kind of pillars and the rear echolons as reserves ready to jump into the line where ever the Hastati could not be brought to a halt when passing on the Principes.
Another possibilty would be that the Principes were advanced maniple by maniple replacing single Hastati maniples that had been exhausted or hacked to pieces by the enemy. This might also happen when the Hastati had been able to force the enemy to fall back and the short break in the fighting was used to exchange maniples, either one-by-one or the entire line.
I also think the Romans did not fight with gaps in their line and that the usual display of a deployed Legion is wrong:
As you can see the most left column of the Hastati is not supported by Principes while the Principes on the most right are isolated from the rest of the fromation. It also takes to many manouvers to get the rear echolons into the front line:
1. "Maniple, turn right!"
2. "Forward march!"
3. "Maniple, halt!"
4. "Maniple, turn left!"
5. "Forward march!"
6. "Maniple, halt!"
That way each maniple of Hastati is supported by a maniple of Principes and each maniple of Principes is covered by a maniple of Hastati. And forming lines is way eayser:
1. "Forward march!"
2. "Maniple, halt!"
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