Instrument-specific practice is the recognition that "technique" isn't one single, transferable skill a musician builds in the abstract, then applies to whatever instrument happens to be in their hands. Technical facility on a given instrument depends on a genuinely different physical mechanism for each instrument family, correct fingering and pedalling for a pianist, precise bowing and left-hand shifting for a string player, controlled embouchure and breath support for a wind player, and a practice drill that builds real, measurable skill for one of these mechanisms can transfer almost nothing to another.
Each instrument family routes technical skill through its own physical mechanism
A pianist's technical development centres on finger independence, hand position and pedal timing, none of which involves the breath control a wind player spends years developing, or the exact, continuously variable pitch control a string player has to manage with an unfretted fingerboard, where there's no physical marker telling the hand it's landed in the right place. A brass player's embouchure, the precise, sustained muscular control of lips and facial muscles against a mouthpiece, has no real equivalent on a piano or a percussion instrument at all. Each of these mechanisms took the instrument's own historical design, not a generic idea of "musical skill," and asks for a genuinely different kind of practised physical control.
A well-designed technical routine has to target that specific mechanism directly
Because the underlying physical mechanism differs so completely between instrument families, an effective technical practice routine, scales, arpeggios, long tones, bowing patterns, has to be built around the specific muscular and coordinative demand that instrument actually makes, not around a generic notion of "practising technique" that's assumed to transfer automatically. A violinist drilling bow control at the frog and the tip is training something a pianist's daily scale practice simply doesn't touch, and vice versa, which is exactly why serious instrumental teaching is built instrument by instrument, with its own specific technical vocabulary and its own specific sequence of exercises, rather than around one universal technique curriculum every musician works through in the same way.
What we're still unsure about
That technical facility depends on instrument-specific physical mechanisms, and that effective practice routines have to be built around that specific mechanism rather than a generic notion of technique, are well established, extensively taught principles among instrumental pedagogues. What's more genuinely a matter of ongoing pedagogical judgement is exactly how much genuinely transferable benefit a musician gets from learning a second instrument in the same family, a violinist picking up viola, say, versus one in a genuinely different family, since teachers continue to hold real, sometimes differing views about how much of that technical investment actually carries over versus how much has to be rebuilt close to from scratch, rather than there being one settled, universally agreed answer.
This sits inside Technique & Instrument-Specific Practice, one of seven topics in Performance, one of four domains in Music, one of seventeen subjects the app can quiz you on.