I am running some tests on the account method .move_char, but I cannot see how it could work properly.
Logically to make this work you need
source account,
source index,
destination account,
destination index.

The actual method and parameters are:
Account.move_char(string destacctname, integer slot);

In other words there is only one index..... and it doesn't say whether it's source or destination.

Can I have a fuller explanation of this please?
My tests show that it is the source index, not the destination index. In which case how do you tell it where to put it? Does it just take the lowest available slot 0 - 4?
The core-changes says "you can use it to flip chars on same account too" but then I have to ask what is it doing?

In my opinion its source slot.
account.move_char(destination_account, source slot);

I think that it will put it on first free slot on destination account, in example:
Destination account:
- Franek Koleczka (1)
- Empty slot (2)
- Zbyszek Zapalniczka (3)
- Zenio Smietanka (4)

acc.move_char(acc, 4);

Destination account:
- Franek Koleczka (1)
- Zenio Smietanka (2)
- Zbyszek Zapalniczka (3)

I was useing this function, but long time ago and i dont left any notes in my transfer script so i can be wrong.

I am using the command regularly and it does sent the character to the next available slot.