there is a full (plain TeX) example and screenshots in a (non)answer on stackexchange
http://tex.stackexchange.com/a/354485 but in brief: \left| T \right| has a kern between the T and the right hand | matching the kern in |T| if tfm fonts are used in luatex or xetex or pdftex but if Opentype Latin modern is used xetex shows the kern but luatex has no kern once \right or \Uright are used, but does still show the kern with |T|. I'm not sure if this is a luatex or luaotfload issue? David
