A very serviceable answer was provided here by /u/Iphikrates. Having gotten the "real" answer out of the way I should like to add some remarks. I find all these debates about spear usage, on the internet and even in academia, rather silly, because they usually betray a general lack of understanding of how actually to manipulate the weapon. People generally don't seem to understand the way spears work. They're not bladed spearpoints floating several feet in front of one's hand manipulated by hand and arm strength. They're six or nine foot long levers that, apart from being quite long, are fantastic force multipliers when used properly (in this I must disagree somewhat slightly from /u/Iphikrates, in that if the main thrust--heh--of a spear was its length alone it would not be an especially effective weapon. Its length allows any force put into it at any angle to be multiplied quite enormously, making it by far the most forceful of all the "standard" weapons). A spear, properly weighted or not, must always be manipulated with the core--indeed, if used one-handed and overarm the angle of the elbow should not actually move much at all. One of the exercises I give my students to teach them proper spearwork (two-handed usually in Okinawan and Japanese martial arts, but as I always demonstrate to them the front hand is really just there to guide the weapon somewhat better and nearly everything done with two hands can be done by someone with even a little training with one) is mentioned there, but to explain better the weapon is set in motion--no matter whether underarm or overarm, two-handed or one--from the ground up, particularly with the counter-rotation of the hips and the use of the core. Any other way of manipulating the weapon results in the arm doing most of the work, and the human arm simply cannot provide either the requisite power or control that the rest of the body can. It takes relatively little training to get the hang of when taught properly, as it's quite a natural use of the body's muscles--after only a couple hours all but my most difficult students are usually able to strike targets accurately and with enough force to penetrate (with a sharpened weapon) both underarm and overarm and with either one or two handed strikes.