Let's assume you have gotten enough Tocharian under your belt that you're ready to move on to other texts. Chances are, you're reading a Tocharian B text; so you place a document with the text on your desk, and beside it your steady companion, the venerable Dictionary of Tocharian B by Douglas Q. Adams. As you read, you come across the word B kelu. You have no recollection of this word, so you look it up in your dictionary. Interestingly, you find there is no listing for kelu, not even a hint like the helpful 'q.v. ...' telling you where else you might look. Nothing. All you find in the near void of kel... entries is a word for 'bellybutton' and some term for 'a medical ingredient'. Neither of these has a chance of giving you kelu, so you may wonder if it was really worth all the trouble learning this esoteric language in the first place. Now what? Search page by page through the section on k? Do you realize how many words start with k in Tocharian? No...
Your only sensible option is to devise some way of cutting down your work and honing in on some realistic possibilities. How to proceed? First, consider what you do know: kelu is not a headword. That is, it is not an adverb; it is not the nominative singular of a substantive; and it is not a verb form transparently displaying the basic verbal root from which it derives. So it must be some other inflected form: either a non-nominative case form, or one of the many possible conjugational forms of a verbal root. At this point, you may as well guess. The final -u of kelu reminds you of the preterite participle, and so you guess that what you're confronted with is a verb form -- maybe even a preterite participle, if you're lucky. But from what verb?
This is where the extra time you spent learning Tocharian in light of historical linguistics finally pays off. You reason as follows: the initial k- is probably part of the root, since there are few changes in Tocharian that will actually produce a k- from some other sound. The same reasoning might convince you that the -l- should be in the root as well, unless this is a gerundive; but if it were, you'd be at a loss to explain the final -u as an ending. So you decide that the root is something like k-l-: that is, maybe there's something between the k- and the -l-, but maybe not; perhaps something comes after... You'll simply remain non-committal. What could sit between the k- and the -l-?
To answer this, you ask yourself a simple question: what produces -e- in Tocharian B? You recall PToch *æ, of course! But that just begs the question: what produces PToch *æ? There you have two options: PIE *ē or PIE *o. How do you decide? Well, suppose the -e- in kelu came from PIE *ē. Then the evolution PIE *ē > PToch *yæ means that the k- should be palatalized, which it obviously is not. So the -e- you see before you most likely comes from PIE *-o-, and you can safely assume that you're trying to find the verb to which a form like PIE *kol- would fit somewhere in the paradigm. But where would that crop up?
Well, you've already assumed you're looking at a verbal root, and *kol- looks like a good candidate for PIE o-grade. The other major forms should derive from either PIE e-grade or PIE 0-grade. One of those probably yields the Tocharian form of the root you're looking for. But which? Well, if it were PIE e-grade, you'd have to consider the evolution of *kel-, and you immediately realize that *-e- will palatalize the *k-, and you shouldn't be looking under k at all. But if you had PIE 0-grade, then you'd have PIE *kl- > PToch *käl- > B käl-.
So armed with your new guess, B käl-, you return to Adams' dictionary, only to find there are two distinct roots with that shape. But, happily, under the first one you chance upon the form kelu as the first person singular active of the CLASS i subjunctive. Did you say subjunctive? CLASS i? You should've known! You figured it was PIE o-grade, after all, and that fits exactly with the development of subjunctive CLASS i. But whatever, you've found it! ... and all as a result of the techniques of historical linguistics that seemed like such hocus pocus until now....
As an added hint, realize that you don't have to follow through the process flawlessly for it to be helpful. For example, who's to say at the beginning of our discussion that we shouldn't have guessed the Tocharian B k- came from an original PIE *g-? Fine, go with it. You would follow the same procedure as above, looking at different grades of a PIE root *gel- (?); but in the end, when it comes back down into Tocharian in the PIE 0-grade, that *g- will still become k-, and you'll still get to the dictionary entry for B käl-. (In fact, Adams lists *kʷel- as the original PIE parent of the verb we're looking at.) Or you might guess some other consonant and different vowel grade -- that's fine, since even following all the different vowel grades of three or four PIE roots (allowing for different sources for the initial consonant) will still leave you with less work than trying to read through the entire k-section of the dictionary entry by entry. So may you enjoy your newfound understanding of Tocharian, and may your knowledge of basic historical linguistics keep you in good stead.