First, my own example.
With only N-S vulnerable, I was sitting West at matchpoints when South dealt and opened 1NT (15-17). I passed, North bid 2 hearts, and South transferred to two spades, all pass.
Holding ♠ AQ7 ♡ A9863 ♢ T8 ♣ T54, I led the fourth highest heart. South said (after the game), "You shouldn't have done that. What if I had a singleton king?" While this was just barely possible, I doubted that because of his 1NT bid.
Here's what I was thinking:
North had shown great weakness with her bidding sequence, but she had most of her side's trumps, which would be exposed on the board.
Because of 1, above, my partner and I had half the points in the deck, with the need to take only six tricks to defeat the contract. I expected to (and actually found) honors with my partner, who held QJ stiff, and was able to ruff a third heart. Moreover, we had at least equal strength, though not length in spades.
While the contract was technically a suit contract, the strong South hand was actually a no trump hand.
Because of all of the above, I wanted to start a "dog fight," playing this as a quasi no trump game.
The other lead I considered was a low trump. I didn't fancy leading one of my weak minor suit holdings.
Here's an expert example (Frank Stewart). South gets to five spades after he and North bid three suits, and find that they are off the AK of the fourth suit, clubs. So West underled his ace of clubs, East put up the king, returned his last club to West's ace, and they won the setting trick with a club lead from West, ruffed by East.
Surely, the expert West did the right, or at least a reasonable thing. Did I (a non-expert) do so by underleading an ace? If not, how does my situation differ from the Stewart example.