What's Preventing Query Folding in this Power Query?

Question

I've watched the Power Query presentation at the 2014 Tech Ed, and I'm trying to understand why this PQ (below) doesn't employ query folding.  The Tech Ed Power Point presentation states suggests that "Internal Power Query logic" affects folding,
but I'm unclear what that means.  Are the Text functions inhibiting query folding?  If so, how might I "scrub" arguments in a custom function parameter list while still encouraging query folding?
let
Source = Teradata.Database("fsltdprd.am.freescale.net"),
Product = Source{[Schema="EDW",Item="PROD_PDM_EFF_CURR"]}[Data],
Prodline = Source{[Schema="EDW",Item="PROD_LN_PDM_DOC"]}[Data],
ProdFilter = Table.SelectRows(Product,
each [MTRL_TYPE_CD] = Text.Upper(Text.Trim("AA"))
and Date.From([REVSN_RLSE_DTTM]) >= #date(2014,1,1)),
PrdlFilter = Table.SelectRows(Prodline,
each [TM_VAR_STS_FLG] = "A"
and [EFF_TO_DT] = #date(3999,12,31)
and Text.StartsWith([PTI_CD],Text.Upper(Text.Trim("J")))),
ProdCols = Table.SelectColumns(ProdFilter,
{"PART_ID","MTRL_TYPE_CD","PLC_CD","PAO_CD","REVSN_RLSE_DTTM","PROD_LN_ID"}),
PrdlCols = Table.SelectColumns(PrdlFilter, {"DOC_ID","PTI_CD","PKG_CD"}),
Join1 = Table.Join(ProdCols, {"PROD_LN_ID"}, PrdlCols, {"DOC_ID"}, JoinKind.Inner),
Results = Table.RemoveColumns(Join1, {"DOC_ID"})
in
Results

Answer 1

Btw, this is the "finished" product ... 
(optional #"Pti 1 thru 4" as text
, optional #"Mtrl Type" as text
, optional #"Last Revision Dttm" as datetime) =>
let
Source = Teradata.Database("fsltdprd.am.freescale.net"),
MtrlType = Text.Upper(Text.Trim(#"Mtrl Type")),
PtiCode = Text.Upper(Text.Trim(#"Pti 1 thru 4")),
LRevDttm = #"Last Revision Dttm",
Product = Source{[Schema="EDW",Item="PROD_PDM_EFF_CURR"]}[Data],
Prodline = Source{[Schema="EDW",Item="PROD_LN_PDM_DOC"]}[Data],
ProdFilter = Table.SelectRows(Product,
each (if MtrlType = null then true else [MTRL_TYPE_CD] = MtrlType )
and (if LRevDttm = null then true else [REVSN_RLSE_DTTM] >= LRevDttm )),
PrdlFilter = Table.SelectRows(Prodline,
each [TM_VAR_STS_FLG] = "A"
and [EFF_TO_DT] = #date(3999,12,31)
and (if PtiCode = null then true else Text.StartsWith([PTI_CD], PtiCode ))),
ProdCols = Table.SelectColumns(ProdFilter,
{"PART_ID","MTRL_TYPE_CD","PLC_CD","PAO_CD","REVSN_RLSE_DTTM","PROD_LN_ID"}),
PrdlCols = Table.SelectColumns(PrdlFilter, {"DOC_ID","PTI_CD","PKG_CD"}),
Join1 = Table.Join(ProdCols, {"PROD_LN_ID"}, PrdlCols, {"DOC_ID"}, JoinKind.Inner),
Results = Table.RemoveColumns(Join1, {"DOC_ID"})
in
Results