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“Magic” to put it simply. But for your specific instance previous details that went entirely absent become applicable, at least until the spell ends or the user chooses to utilize them “normally”.

The tension of a crossbow is indeed where all of the damage comes from, and crossbows that aren’t made with extensive mechanical systems still require much strength to operate. Perhaps not as much as say, a bow, but certainly not as simple as “load arrow, aim, shoot” that most people assume with crossbows. Also crossbows [i]do[/i] have recoil, something most everyone neglects to notice or remember. So a strength based crossbow will be one that utilizes magic to draw out more force from a weapon but requires actual more physical exertion from its user, with an equally notable more recoil from the weapon. And unlike your usual range weapon woth the implication that you’re aiming your bolt towards a weak point in the enemy’s armor, the crossbow bolt will instead be launch with sheer penetrative force.

Effectively gives a knife a sort of light saber whip effect. If that seems confusing congratulations, that is exactly how such a spell can often befuddle an opponent who doesn’t know what the spell is, or even an ally who isn’t used to having their weapon capable of such a thing. After all most people are not designed around the abilities this spell can provide.

Conversely a reach weapon will end up shorten. Whips will be little more than a floppy club, and most polearms basically become a slightly weaker greataxe or particularly heavy spear in the case of the pike. 

Light weapons who lose their light property become balanced very differently in a notable way to reflect whatever their new property would be. A dagger that loses its light property for reach is still one pound, but now it’s entire weight is being distributed on a weapon as long as a whip without nearly the same level of design or balancing, making it unwieldy for your usual two-weapon purposes despite no effective change in weight. Conversely, replacing a feature like say, the two-handed feature for the halberd first the light feature, still makes the halberd the same weight as before. In this instance the magic of the spell will being doing most of the work to make the weapon “feel” balanced for one-handed use, which again can be awkward for people who aren’t used to it but not to the point they suddenly lose proficiency.

What is actually happening is simply magic. I created this spell because the alternative of simply reintroducing things like compound bows or long spears meant to be used with shields have so far never been accepted whenever I propose them, despite whatever evidence that exist in real life which support them. “It uses magic” has so far been easier to approve of these basic weapon variants since the implications that surely these weapons don’t exist normally in these worlds, and if they do they’re not meant for players to utilize, otherwise they’re be ground breaking inventions that changes the whole meta, or other similar knee-jerk reactions to the established setting.

A guy with a eight-foot long spear and a shield goes back further than medieval  times all the way to the classical era to the point most people associate that style with any Greek warrior who isn’t simply Hercules himself, but by raw mechanics that’s impossible for any player to replicate in game barring the divine intervention known as “homebrew”. Hell its easier to be Hercules, a demigod of Zeus, than it is to be a Spartan Warrior with a big shield and a long spear. 

Every elf and their mothers have a dexterity bonus and an inexplicable talent to use a longbow, but rarely would any of them also have the fucked up skeletal and muscular structure real human longbow men had due to all of the core strength they’re practically raised to have in order to make the most out of those weapons, with the only explanation being “they’re elves” and “longbows use dexterity not strength”.