http://www.tcl.tk/man/tcl8.6/TclCmd/transchan.htm
This is a protocol for defining a channel transform, which is a way to intercept the reading and writing of bytes on a channel with a Tcl script and modify them (e.g., handling encryption, logging or compression). The script is attached via chan push.
[replace with details of this command]
[DKF: It's not a command; it's a protocol that a command prefix should adhere to to participate as a channel transform.]