So many Laerteses will broadcast their villainy to the rafters in this moment. They may as well give an evil laugh and wiggle their fingers, it is so obvious that they are up to no good. (Sidebar: two adorable children gave me a tutorial on how to be a “mean guy” a couple of days ago. It involved smiling maniacally and wiggling my fingers.) But it is rather a practical problem that Laertes has here. He has sharpened and poisoned only one sword and if the sword hander-outer hands him NOT that sword, then all his plans of murder are spoiled. In fact, if, by chance, the hander-outer hands Hamlet the sharp poisoned sword, he’ll be in trouble. (I mean, he gets hit with it anyway – but that’s a problem for later.)
Given that Osric is handing out the swords, one might start to wonder if he’s in on it. If he IS in on it, then this bit about the heavy sword is all performance. It would suggest that they’ve thought to make a little production of choosing the poisoned sword accidentally.
If Osric ISN’T in on it, Laertes has to work out how to get the sword he needs for the killing job. He probably has to point to the one he wants while he distracts everyone.