Specifically, halberds, pikes, and lances. Essentially, the goal is that if you hold the brace button (which would probably be a new button. Let's say... R) you level your weapon. When on foot this will cause a speed debuff, but if something or someone touches your weapon while it is braced they take damage. Not the full weapon damage, and only once (so you wouldn't deal huge amounts of damage), but modified by your relative speed.
That way, cavalry charging into pikes would be shredded (because reach and speed damage) but lancers would destroy normal infantry (because, again, reach and massive speed damage).
Think Total War, or Mount and Blade.