There’s an interesting thread on Facebook on this topic.
Most would consider cast handstand in a hollow / stretched position throughout as ideal. Nick Blanton mentioned this illustration from Championship Gymnastics.
It’s possible to do that technique. In fact it’s fairly easy on Parallel Bars as a swing handstand.
But I’m personally a very happy coach if the gymnast can lead with the heels (arch) to horizontal, then change the body position to hollow / stretched.
The ultimate would be perfectly straight through hips / trunk once leaving the rail. That’s difficult. The gymnast would likely be strong enough to do planche press without swing.