For posterity, I'm sure this is about the Foot Knights set (as the only set in this range at this moment), and if so:
I'm pretty sure the back of any knight's spare left fist would attach at the "X" -shaped straps on the inside of the shield, While any straight "I" -shaped straps would lay under the forearm.
Having seen a number of similar kits now, I'd suggest that it's not usually "rocket surgery"on the fist for this sort of thing: the shields are interchangeable with any of the loose left arms (especially those with fists instead of open hands). the shields hide most of the arm any way (most competing kits have fairly rudimentary arms!), and the shelds can just glue onto the arms with no special fit (there are rarely any particular tabs or slots or anything to fit shields to arms in these sorts of kits.)
With this kit (and other Wargames Atlantic sets with shields), you're in somewhat better shape than you would be with other manufacturers (Oathmark and Frostgrave comes to mind), in that the nice detail on the loose left arms means that you don't need to hide any crude, strange-looking arms behind shields (the arms pre-attached to torsoes are a different story!), and so you are free to leave sheilds off of the some of the figures if you like, or mount the shields on their backs, or whatever else you like, and you're also free to swap the kite shields included with the knights for a different style (I'm sure some round shields would look great on these guys, for use in fantasy games at least!)
Those arms that are molded onto the bodies do look just strange enough that I think I'd want to hide them behind shields, though! (YMMV)
Anyway, no bump needed for those loose arms to fit into the shields - you can glue the shields onto these arms however they look best to you, but I'm pretty sure the X-shaped straps are technically where the hand would hold the shield, and the I-shaped straps are the ones that would cross the fore-arms.
Here's a reference image that might help you a bit with the placement: