While I understand that people might find their flashy jumping on horseback and shooting from a crouched position appealing, I found it rather tacky, gimmicky and not at all as visually appealing as the show's creators would like to think.
ah its simple, standing on stirrup and shoot is actually more practical and historically accurate than standing on the saddle.. on galloping speed. Though i must say, in manguds and Tarkhán tactic and rank, switching seating position of facing back and front is described actually..just not in the same style as dothraki screamer like the one hbo try to potray.
some horse archer use that kind of method if they shoot a heavy draw weight bow and find their ussual back muscle-tricep and horizontal elbow drawing a bit lacking in strength. Like...drawing 180lb composite bow past the back of head
this is war screamer, not circuss!