“Body like a mountain” invites you to feel what effortless sitting is actually like. When I let the words sink in without trying to analyse them, the body knows what to do. The meaning of the metaphor reveals itself in practice.
Ken uses metaphors because they help us bypass the urge to figure things out, and take us straight into direct experience. That emphasis on direct experience is a hallmark of his approach.
From Pointing Out Instructions 3
Ken: A lot of instruction in Buddhism is expressed in mythic language. I'll give you one example. It happens to be a mahamudra instruction—I thought I'd keep it on topic.
Body like a mountain,
Breath like the wind,
Mind like the sky.
Now, I think this is quite a good example. If I say body like a mountain, what do most of you think of immediately?
Student: Rigid.
Ken: Yeah. Okay, now what's it's like meditating like that?
Student: Tiring
Ken: Does it work? No. So, is this what it means?
Student: No.
Ken: No. So, what does it mean? Nick?
Nick: You become very stable.
Ken: How do you become very stable? Can you just say "I'm going to be stable."? Does that work?
Nick: You relax your body.
Ken: What does that have to do with a mountain?
Nick: Mountains don't make any effort to sit there like that.
Ken: Exactly. Say it again loudly.
Nick: Sorry. I said mountains don't make any effort to sit like that.
Ken: Yeah. So this phrase—body like a mountain—means to sit without any effort whatsoever. And you come at this by actually just taking it in and letting it speak to you, not trying to analyse it and figure out exactly what it means, etc., etc. Okay.
This is the language of poetry. And it's similar things with
Breath like the wind,
Mind like the sky.
So, in what we work with in these days that we're here together let the instructions sit in you. Let them reveal their meaning. You'll know when they've revealed their meaning by what happens in your practice.