I'm reading 60yoc's stuff.

He mentions its on moments and hand caressing and suggests you can either hold out your hand, high five and hold on, or hold on after a handshake.

These sort of perplexes me. So you just go around shaking girls' hands w/ seductive eye contact and then started caressing their hands- wtf?

There must be situational evidence and indicators (maybe waiting for IOI's? but if you do it right after handshake you might not see any aside from her matching your seductive eye contact) of when it is a good idea to do this.

I mean that can't mean tomorrow in biology class or at a party I should just caress every girl's hand I shake can it?

What you can do aswell and will give you the same result is shaking hands and holding her hand just that little too long. While doing that you look straight into her eyes, slightly seductively. She'll feel what you're on about.

You don't "hold" on. After a handshake, simply don't pull away first. If anything she pulls away and you let her hand go and it ends up being just a handshake. If she doesn't pull away you can transition this into hand caressing.

Don't forget to keep talking while you're shaking hands, otherwise it's just weird and not seductive. You don't want the fact that you're not pulling away first to be the center of the interaction until it becomes mutual hand caressing