One of the least noticed,
but most powerful,
body signals is given by the human palm when giving someone directions or commands and in handshaking
palm facing up
- submissive
- non- threatening
- holds no weapons
- you're expecting them to talk
- you're ready to listen

palm facing down
- project immediate authority
- giving order
- if a person is your subordinate,
the palm down gesture is seen acceptable because you have the authority to use it

when a power player presents you with a palm down gesture,
repond with your hand in the palm up position
then put your left hand over his right to form a double hander and straighten the handshake.
double handshake also tries to give the impression he is trustworthy and honest.
it's also known as 'politican's handshake'.
but if used not a person he just met,
it can have reverse effect.
the receiver feels suspicious about initiator's intentions.

if you feel the power player is purposefully trying to to intimidate,
grasp his hands on top and then shake it.
do it only as a last resort.
Summary
keeping the palms held vertical and
matching the other person's grip is usually perceived as a 10/10 handshake.