"Swingy" is a term some gamers use for a rule or a system that can make conflicts go all the way for one side or all the way for the other, based on a die roll or some other consideration. Notably, swingy systems don't allow the players much opportunity to influence the outcome once it's known which side is winning, and often don't allow an opportunity to prevent or influence it beforehand without some kind of GM fiat.
Case in point is Tunnels and Trolls' combat spell Take That You Fiend. It does the caster's INT stat worth of damage directly to the opponents CON/Monster Rating, disregarding armor. And though the spell description states that "...it is a singular spell and must be directed at a single foe", it also has a disruptive side-effect that adds to the caster's party's combat total. It can't do double damage, but it can pad the numbers so you don't take any. Other spells, like Hellbomb Burst, simply destroy everything in their range. But at least it's a higher-level spell.
Some in-game rules will negate this, such as possessing a weapon made of meteoric iron, which cancels 3rd level and lower spells. But even then, a higher-level wizard can just cast a 4th-level Take That You Fiend. The 7th edition also introduced a new attribute called Wizardry, which is a rating of your character's affinity for magic; spells cast on a character by a wizard with a lower Wizardry rating simply do not take effect, which is why even non-spell-casting characters would want to boost that stat.
As you can see, very "swingy".
In light of this, I propose a new rule for games where the GM and players wish to moderate the power of combat spells: after having a damaging spell cast on a character, the player can roll a SR (Saving Roll, based on Luck or another of the character's stats); the difficulty level is the level of the spell caster. For every point over what is needed for success, reduce the damage by one point. The "minimum roll" applies for success or failure, but not for the amount calculated.
If that last bit sounds confusing, consider that T&T requires a minimum roll of 5 (on 2d6), regardless of the attribute or SR level. A 2nd-level SR, for instance, requires a total of 25 points (attribute plus roll); so if Len the Lucky (with a Luck of 23) needs to make a 2nd-level roll, he'd only need two points on two dice--but there's always a minimum of five needed. If he rolls, say, a nine in this scenario, he'd subtract seven points, not four. Failing the roll wouldn't help him at all, though.
Alternately, you can let the magic system work as-is in most cases, but in any case where the spell would kill your character, you get to make the SR and keep as many points of CON as you made the roll by.
This alone is probably not enough to ease the "all-or-nothing" feeling of some T&T magic. It's possible that some spells (like the aforementioned Hellbomb Burst) need to be rewritten or discarded for this style of play. (Please take note of that last clause: if you like T&T's magic as-is, I'm not saying your fun is Wrong and Bad. I just like my wizards to have less of a combat role, because that's where I like warriors to shine.) I'll delve into that a bit more in future posts.