You lead the A and partner plays the 3. Is this 3 significant? Does it mean to shift to clubs?
Only if you play bridge at a reasonably high level. Otherwise why not play the card face down on the table? As long As your signal has no meaning, why bother letting partner see it?
The point is that if you trust your partner (not easy these days), you should shift to a club after cashing the A. If South has a two-way club guess for the queen, partner (East) should have signalled a high diamond at trick one - which means, believe it or not, don't shift, just continue.
Any way, the expert Wests who did lead the A did not shift to clubs, but continued diamonds in face of partner's low diamond. Now declarer could discard a club, cash the K, draw trumps ending in dummy, ruff a heart, and return to dummy via the A for three more pitches.
If West had shifted to a club, that A would not have been there for the final entry.
Bridge Today, Sept-Oct 1992, p. 18, by Matthew Granovetter