The mechanics I've described up to this point work well for slow to moderate tempos, but they run into difficulties at fast ones. Basically, there's too much to think about, and we need to simplfy.
Ornaments exist to give shape to melodies, they should not all be the same and, in Bulgarian accordion, they aren't. Up until now, I've suggested executing prall-triples as 3 even counts. On count 2, finger 3 brushes the auxillary note, finger 2 mometarily comes off the key and then returns. One of the reasons this sounds interesting is the very brief moment when both the base and auxillary sound (after the finger 3 brush but before finger 2 is raised). As with the double-note hold onset, the simultaneous minor second jumps out at the listener, although in a prall-triplet the discord duration is extremely short. As a result, the prall doesn't sound discordant, just interesting. Those new to Bulgarian accordion (especially those who focus on learning by notation) may play pralls like classical triplets. Apart from the timing problem, the lack of the momentary discordant 2nd makes such ornaments sound lifeless and blah.
An alternative to the even 1-2-3 counting of a prall-triplet is to rush count 2, starting the finger 3 brush early. This is accomplished by starting finger 3 in motion before finger 2 has touched the base note. The result is fingers 2 and 3 are "thrown" toward the ornament in a single action, with finger 3 slightly following finger 2. After finger 3 depresses, the prall completes with a return to the base note on finger 2 as before. All 3 notes of the prall get clustered toward the front end of the 8th note the prall occupies, and the simultaneous sounding of the base and auxillary notes is somewhat more prominent than before, resulting in a sharp, percussive effect that jumps out at the listener. I call this a "sharp" prall, as contrasted with the "smooth" pralls previously discussed. Note that in sharp pralls, the 1st and 2nd notes of prall are temporally closer together than the 3rd note, while in smooth pralls the 2nd and 3rd notes are closer together than the 1st note.
Sharp prall produce a more aggressive sound that can be useful in shaping melodies (even at slower tempos), but their chief utility is that at fast tempos, where they can be more easily executed than smooth pralls. The two actions that comprise the smooth prall are blended into one, simplifing execution and your thought process.
It is tempting to think of a sharp prall as a double bounce of your hand, the first on the base and auxillary notes, and second only on the base. I'm undecided at present whether this image is helpful to good execution not not, so experiement and let me know what you think.
Here's a classic Boris Karlov passage to practice this technique. As always, slow, relaxed practice is preferred (that can be especially hard to maintain here because this technique generates a lot of excitement). First play the passage with even 1-2-3 timing a few times, and then try executing the count 2 finger 3 brush early, and then back to even again. When playing "sharp" ornaments, strive for a "crunchy" sound.
This manner of practice can be repeated with any of the previous melodies and exercises.
Trills comprising 3 16ths (3 base notes, 2 auxillaries) benefit from the "sharp" methodology even more than pralls. I find very fast tunes that use a lot of such trills (e.g. elenino horo, graovsko) impossible to execute at tempo using "smooth" mechanics. In a sharp trill, execution of the 2nd auxillary bears the same relation to the 2nd base note as the 1st auxillary bears to the 1st base note - i.e. it is hurried relative to a smooth thrill, and executed by starting finger 3 in motion before finger 2 has depressed the base note.
If you find the double-bounce imagery to be helpful for sharp pralls, you will probably find triple-bounce imagery to be helpful for sharp trills.
The following melody from the classic Elenino horo make for good sharp trill practice:
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