Around 400BC in Nemea, the Greek fighters Creugas and Damoxenus fought until dusk.
They resolved to permit each other a single final blow.
Creugas struck Damoxenus on the head but the blow was weathered.
Damoxenus had Creugas raise his arm and then struck him with his open hand, using his fingertips like a spear.
Damoxenus penetrated his torso, killing Creugas.
Many other fighting styles include techniques for striking with the finger-tips.
Spear-hand is a thrusting strike executed with the tips of the stiffened fingers.
Potentially the spear-hand is a very effective technique that can deliver a lot of force into a very small area.
It is a good hand form for hitting small, hard to access but very vulnerable targets.
In practice spear-hand is often neglected by martial artists since it cannot be used when wearing sparring gloves and many of its targets are not legal or too dangerous to attack in sporting competition.
Another reason for its neglect is that developing a strong spear-hand requires more conditioning and training than most other anatomical weapons.
Supposedly the way to condition your hand(s) for spear-hand is to thrust your hands into a bowl of powder or fine sand. Once used to this, you work up to coarse sand, dried beans, gravel, ball bearings, and, if some movies or books are to be believed, hot rocks or coals.
In GURPS Martial Arts (4e) rules for the result of this conditioning are on p.42 and p.50. Hands so conditioned have a DR1  and are treated as blunt claws  (p.B42). Conditioning to achieve this takes 600 hours and comes with the disadvantage Bad Grip 1 [-5] (p.B123).
In Transhuman Space, a similar effect can be gained by a relatively minor biomodification ($5,000 and 15 days recovery).
Placing the thumb-tip on the second joint of the forefinger causes the fingers and wrist to automatically lock in the optimum configuration for powerful spear-hand strikes.
Treat this as an equivalent to the blunt claws advantage (+1 damage per die) for thrusting attacks made with the hands.
This biomod has no effect on the fighter’s grip or dexterity nor does it increase the DR of the hand. The hand appears unmodified. Actually having blunt claws has the same effect but alters the appearance of the hand.
Having sharp claws or talons may increase damage from a spear-hand attack and change it to impaling damage, as per p.B42.
Using a spear-hand strike itself requires the fighter to know the technique Exotic Hand Strike (MA 4e p.71) and/or Lethal Strike (MA 4e p.85).
Exotic Hand Strike is Average and does thrust crushing damage plus Karate bonuses.
Lethal Strike is Hard and gives thrust-2 piercing damage plus Karate bonuses.
Iron-hand conditioning or the Spear-hand biomod add +1 for each damage die.
Spear-hand requires the striking hand to be empty. Spear-hand techniques get no modifier for brass knuckles and cannot be used with boxing gloves etc.
Karatand gloves are made of a memory plastic that becomes rigid on impact (UT 4e p.163).
Karatand gloves are described as having the same effect as brass knuckles which seems to suggest they add +1 to total damage, making a punch thr cr rather than thr-1 cr.
It seems reasonable that the rigid fingers of a Karatand would give +1 to spear-hand strikes.