Puzzled By Query Plan

I'm hoping somebody can explain exactly what's going on here - I can't
find it documented anywhere.

Go to the Northwind database, and run the following SQL:

create index IX_UnitPrice on [order details](unitprice)

Now, turn on SHOWPLAN (either graphical or text, it doesn't matter),
and run the following query:

select * from [order details]
where unitprice = 2

Output:

StmtText
|--Index Seek(OBJECT: ([Northwind].[dbo].[Order
Details].[IX_UnitPrice]), SEEK: ([Order
Details].[UnitPrice]=Convert([@1])) ORDERED FORWARD)


Now, alter the SARG slightly by making it a float:

select unitprice from [order details]
where unitprice = 2.000

Output:

StmtText
|--Nested Loops(Inner Join, OUTER REFERENCES: ([Expr1003], [Expr1004],
[Expr1005]))
|--Compute Scalar(DEFINE: ([Expr1003]=Convert(Convert([@1]))-1.00,
[Expr1004]=Convert(Convert([@1]))+1.00, [Expr1005]=If
(Convert(Convert([@1]))-1.00=NULL) then 0 else 6|If
(Convert(Convert([@1]))+1.00=NULL) then 0 else 10))
| |--Constant Scan
|--Index Seek(OBJECT: ([Northwind].[dbo].[Order
Details].[IX_UnitPrice]), SEEK: ([Order Details].[UnitPrice] >
[Expr1003] AND [Order Details].[UnitPrice] < [Expr1004]), WHERE:
(Convert([Order Details].[UnitPrice])=Convert([@1])) ORDERED FORWARD)


Right. I understand that in both cases the SARG datatype is different
from the column datatype (which is money), and that in the first
example the SARG constant gets implicitly converted from int -> money
(following the datatype hierarchy rules), and so the index can still
be used.

In the second example, the datatype hierarchy dictates that money is
lower than float, so the table column gets implicitly converted from
money -> float, which strictly speaking disallows the use of the index
on that column.

What I DON'T understand is what exactly all that gubbins about the
expressions (especially the definition of [Expr1005] is all about; how
does that statement decide whether Expr1005 is going to be NULL, 6, or
10?

I'm soon going to be giving some worked tutorials on index selection
and use of Showplan to our developers, and being a bolshi lot they're
bound to want to know exactly what all that output means. I'd rather
be able to tell them than to say I don't actually know!

How about it someone?

Thanks,

Phil