Sorry for a late answer to an old question. But some answers suggest that the technique is too dangerous to be used; or that you should bend your fingers to allow safe crumble; or that there's no way to properly condition for use.
These answers assume that there's only one way the spearhand is to be used, which is that it is a power strike into the target.
Why assume that? A gentle backhand smack to the eyes can be enough distraction for the opponent to become off-balanced, lessening the impact of his strike. Surely if a gentle back-smack can do that, a gentle poke to the eyes - even a threat of a strike to the eyes - can do the same, yes?
These answers also assume that the tips of the fingers are what strikes the target.
Why assume that? Can't the palm of the spearhand be used to rub along the attacker's face? If meeting your opponent head-on, if your right hand is the spearhand, your left arm is crossed in an "L" shape in front of you. This is a common defensive movement: you're holding your opponent to you while you slide your hand up the chest, over the face, and beyond: this has the effect of bending his head backward. Isn't the shape of the spearhand the same?
Or are we talking about where the palm is vertical (facing left with a right-hand spearhand). Well, meeting your opponent from the side and issuing a similar strike has the effect of twisting the head sideways, snapping the head laterally, maybe breaking the neck, inducing vertigo, or applying a headlock.
I get it that we see these strikes as a power strike. After all, I break with fingertips. I'm above average with respect to fingertip strikes, given my constant practice for breaking. However, I would never use a fingertip strike as a power strike against an opponent. Rather, I would use it as a distracting movement, or to poke the eyes or throat - but not with full power. Doing that could cause injury to both of us, and, if he's got a friend who can help him, I'm going to be fighting his friend next - and without a good hand to do so.
We should never assume a technique's use based on its name. Giving a thing a name thus limits its use, and that is unfortunate because it makes it difficult for us to imagine more uses. To figure out what a technique's use is, you need to look at many things: what's the other hand doing? What's the stance you're in? What's the previous movement? What's the next movement?
Did you ever see a spearhand being applied WITHOUT the other hand's palm beneath the elbow? If you did, it's probably high - as if attacking the eyes with the finger tips, and the stance was probably a front stance. If it was very low, the palm was probably facing up and it's probably to grab at the groin or knee - but not attacking by the tips of fingers. (and most likely, you're in a back stance or cat stance). Ever wonder why?
Here's another use for a spearhand: it can be a throw.
In Kukkiwon Taekwondo, Taeguek Sa Jang (#4) (https://www.youtube.com/watch?v=zAauWkIZuqY), the first movement is a double knifehand block, then step into a spearhand. The key here is to figure out what the previous double knifehand was doing. If you were competently taught, you'd know you're not blocking anything. Because your hands are knifehands, and not fists, the assumption is that you are being grabbed. The outstretched hands keep your opponent off-balance; if he wriggles into a position most advantageous to him with maximum possibility to attack you, then, he'll jam himself up by having his hands (which are now grabbing you) useless; you step in with your forward hand (the forward part of the double-knife-hand) reach behind his back or head (that's the part that forms the palm down underneath the spearhand), and the rear hand of the double-knife-hand then moves forward over the shoulder, which throws him backward. In Aikido this technique is called "iriminage", and you can see superb examples with Sensei Christian Tissier on youtube (https://www.youtube.com/watch?v=D2EJEWrAxsQ). Hapkido has a very similar throw, in English we just called it a head throw, I don't remember the Korean name.
So there you have it: don't assume a technique's purpose based on it's name. Otherwise, you'll get completely flustered with "Swallow Poom" (which is the high-block/neck chop combo). ;-)
And a word of warning: if you have to condition to use a technique, what good is the technique when you can no longer condition for it? As you get older, the technique that once required conditioning is useless to you. And in many cases, conditioning can be damaging; osteoarthritis is common among people like me who condition for breaking. Yes, I'm paying dearly for it.
And what good is a technique if you are required to condition for it, but you never do condition for it? That's on your instructor, who should have told you that the spearhand need not be a power strike (unless you're breaking). In self-defense, a spearhand is just too dangerous for YOU to use as a power strike.
To answer your question, you ought to explore the many uses of a spear-hand. Don't assume you need full force behind it, such is not the intention. In my belief, a technique which requires such conditioning means to go down a path not intended for the technique. What will you do when you're older, or injured, or fatigued? Learn what a spear-hand really is, and then practice your technique for those cases. If you think hitting a sandbag will improve it, or condition you, then you don't know what it is for. Find a competent instructor who can give you drills, and help you as you practice.