Collé is tricky to learn, and much tricker to describe in words alone! I have to ask the obvious question—are you trying to do this on your own, with no teacher? A good teacher can convey this much more easily than the internet can. And learning the Tchaikovsky concerto without a teacher is a very ambitious goal as well. Don't let me squash your dream, but I urge you to get lessons if at all possible (maybe even talk someone into a single lesson on bow technique!), and to ask yourself whether there are other pieces you need to study to equip you for the Tchaikovsky.
That said, assuming that you already have a solid, healthy grasp of the way all the parts of the arm move in a basic detaché stroke (upper arm, elbow, forearm, wrist), then the finger motion that powers collé is vital to study. It plays a part in almost every other kind of stroke as well, including your basic detaché. Any accent can be made more explosive by it, or it can give an initial note a clean, precise unaccented articulation. (I find it especially helpful on baroque violin to avoid the squeak that gut strings can make when poorly articulated.)
A picture is worth a thousand words when explaining this motion; this video does a pretty good job:
. The idea is that your finger knuckles bend or straighten, letting you pull your fingertips in toward your palm or push them away from it. If your fingers were exactly perpendicular to the stick of the bow, this would simply lift your bow off the string or lower it, but since your hand slants, with your fingers pointing somewhat away from your body, the extension of straightening your fingers moves the bow an inch or so in a down-bow direction, and contracting them moves up-bow. I would practice in these steps:
- Practice the motion without a bow in your hand
- Practice with the bow in your hand to see how it moves. Around 4:30 the video practices this with your left hand keeping your right palm stationary so only your fingers move. I also teach holding the bow vertically, with the screw resting on a table, and using only your fingers to lift it from the table and put it back.
- Put the bow on the strings and use only this finger motion to produce very small up- or down-bow strokes (not yet leaving the string). Don't let yourself move your upper arm or forearm, don't bend your elbow; just the 1-2 inches that your fingers allow you. This will sound terrible.
- Now allow yourself a little bit of arm motion, just enough to turn that useless sound into a useful, accented, detached but on-the-string stroke. Still not removing the bow from the string, use that finger motion to start the stroke and allow a bit of follow-through from your arm.
- At this point, you can incorporate this motion into your everyday detaché by allowing your arm, elbow, and wrist to flow fluidly as usual, and just letting the "squeeze/stretch" finger motion to kick off each bow change.
- To learn true collé, as the video demonstrates well around 2:15, let the up-bow contraction of your fingers lift the bow from the string. This happens very naturally. Getting the bow to lift on a down-bow requires a bit more arm involvement. Vital point: before the next stroke, the bow must be placed back on the string and your arm weight must create friction between the bow and the string. This is true of every stroke except ricochet—ricochet is the one exception in which your bow can come flying in from the air and strike the string without first "gripping" it before pulling—but it's especially important here. Before taking a down- or up-bow, prepare your fingers in the "opposite" position, i.e. contract them before a down-bow so you can extend, or extend them before an up-bow so you can contract.
- Before applying this to the Tchaikovsky passage, which is rapid and includes lots of string crossings, use it for a plain old scale, putting rests between each note so you have time to prepare. Prokofiev's March from The Love for Three Oranges makes an excellent repertoire piece to apply this. It's full of short, sharp, crisp notes that can live on little more than a finger-squeeze. You should also let this motion power the accents that appear within slurs in that piece. Practice this: You're moving in a down-bow direction. Without stopping the bow, allow your fingers to collapse ("squeeze"), then explosively extend them for an accent, and vice-versa on up-bow.
By the way, there's nothing that says that this passage has to be true collé. Joshua Bell in this video certainly gives the notes a length that wouldn't qualify as it:
. That's probably because true collé
is a very crisp, dry stroke, and by definition just doesn't produce that much total sound, and at that moment you're backed by the orchestra in forte; Bell is lengthening to project more (as a soloist often has to). But it's fair to say that this, regardless of whether the bow lifts off the string or how much the arm "follows through" to sustain the note, these notes would benefit from the "explosive" start that this finger motion can provide to the very first milliseconds of each.