The Best Exercises for Hand Synchronization
A mismatch between the fretting hand and the picking hand is heard as a smeared, unclear note, even when both hands are individually fast enough on their own (see [[alternate-picking-basics:the article on alternate picking]]). Synchronization can and should be trained separately from speed.
Exercise one: a single note to a metronome
Take one note and play it in even beats to a metronome, deliberately checking on every hit: did the finger land on the fret at the exact moment of the pick strike, not before or after. At a slow tempo, a mismatch is heard as a double sound - first the finger's thud, then the string's tone - and that double sound is exactly what the drill targets.
Exercise two: chromatics on one string
Play four neighboring frets on one string in a row, one finger per fret (the same principle as the 1-2-3-4 exercise - see the dedicated article), to a metronome at a comfortable tempo. Chromatics remove the need to think about scale notes and leave only the pure mechanics of matching the hands.
Exercise three: string skipping
Play two notes not on neighboring strings but skipping one - this forces the picking hand into a wider travel path, and desync that's invisible on adjacent strings becomes clearly audible. Start very slowly: this is exactly where precision tends to break down first.
Finding exactly where things drift apart
Record a short passage on your phone and listen back separately from playing it (the same trick as in the article on solo phrasing, see the article on solos) - a recording reveals desync far more clearly than playing does, when attention is split between both hands at once.
A slow tempo isn't prep work, it's the actual training
Synchronization built at a slow tempo carries over to a fast one on its own - unlike cleanliness, which has to be re-earned at every new speed, exact hand-matching, once locked in slowly, stays put as tempo rises. There's no reason to rush toward speed before the match becomes fully automatic.
Try it in practice
Open the metronome