It might be a fervent gesture from the heart, but where the fuck did he think the nazis got it from? Or the Romans? I don't think someone sat down calmly and discussed a new salute. Someone getting a bit excited and doing a gesture which people picked up copied is exactly where this kind of thing starts.
There will be people who think that they should model themselves on him and copy it.
It's an outward sign of the over the top, dangerous philosophy that Trump is following in my opinion.