As Matt Ho explained on the mailing list:

A value stack is essentially a List. Calling [1] on the stack,
returns a substack beginning with the element at index 1. It's only
when you call methods on the stack that your actual objects will be
called.

Said another way, let's say I have a value stack that consists of a
model and an action as follows:

[ model, action ]

here's how the following ognl would resolve:

[0] - a CompoundRoot object that contains our stack, [model, action]

[1] - another CompoundRoot that contains only [action]

[0].toString() - calls toString() on the first object in the value
stack (excluding the CompoundRoot) that supports the toString() method

[1].foo - call getFoo() on the first object in the value stack
starting from [OS:action] and excluding the CompoundRoot that supports a
getFoo() method

I hope this doesn't sound too confusing :\

If you're using Velocity, this can most easily be written as:

$stack.findValue("[0]").peek()

Unfortunately, <ww:property value="[0].peek()"/> won't work as this
would translate into "starting at the top of the value stack (and
excluding the CompoundRoot), find the first object that has a method
called peek()"

--------thanks Matt!

here is the com.opensymphony.xwork.util.CompoundRoot class which Matt mentions:

What's on the stack?

NOTE: When rendering Freemarker / Velocity templates or result,
WebWork2 contains the following items by default in the ValueStack:

• req - the current HttpServletRequest
• res - the current HttpServletResponse
• stack - the current OgnlValueStack
• ognl - an instance of OgnlTool
• ui - a (now deprecated) instance of a ui tag renderer

@See com.opensymphony.webwork.views.util.ContextUtil