when use extract transform inside foreach-loop produced unexpected results, Is this a BUG or I mis-understand something?
Video and hip file are attached.

I don't completely understand what you are trying to do, mostly because it seems unnecessary

there are a few issues with your file
- in the portion you are trying ro use Extract Transfrom you are timeshifting the cached piece input, which can't be timeshifted
you should rather save your timeshifted geo's P as an attribute like rest and then restore for each piece to compare with the live one
- but you shouldn't even need to do that since Extract Geometry supports using piece attribute so no for loop is necessary
- extracting transform from straight lines is ambiguous and you can't get stable transform from just 2 points
- using Transform Pieces without having the actual piece attribute like name will use only first transform point
- you are cleaning all the useful attributes like transform and name, is that on purpose and your whole goal is to recreate them? Since you essentially have everything you need to create stable tubes or whatever geo from each bone right at the import

tamte
I don't completely understand what you are trying to do, mostly because it seems unnecessary

there are a few issues with your file
- in the portion you are trying ro use Extract Transfrom you are timeshifting the cached piece input, which can't be timeshifted
you should rather save your timeshifted geo's P as an attribute like rest and then restore for each piece to compare with the live one
- but you shouldn't even need to do that since Extract Geometry supports using piece attribute so no for loop is necessary
- extracting transform from straight lines is ambiguous and you can't get stable transform from just 2 points
- using Transform Pieces without having the actual piece attribute like name will use only first transform point
- you are cleaning all the useful attributes like transform and name, is that on purpose and your whole goal is to recreate them? Since you essentially have everything you need to create stable tubes or whatever geo from each bone right at the import

"extracting transform from straight lines is ambiguous and you can't get stable transform from just 2 points"
YES,I found out that this is not possible,thank u for reply.