twig & penny seemed to drop

Never Let Me Goの一場面です。
'Who was that?' he asked in a bewildered voice.
'You know who it is, Tommy,' I said quickly. It was too risky to kick him, or even to make my voice wink-wink: Chrissie would have picked it up in a flash. So I said it dead straight, with a bit of weariness, like we were all fed up with Tommy forgetting all the time. But this just meant Tommy still didn't twig.
'Someone we knew?'
'Tommy, let's not go through this again,' I said. 'You'll have to have your brains tested.'
At last the
penny seemed to drop, and Tommy shut up.
ここの "twig" の意味が分かりません。 昨日フィギュアスケートの大会で浅田真央選手の滑るのを見たので "twiggy" を思い出しましたが、ここの "twig" は動詞です。　しかし、この後に、上の引用文の最後に "
penny seemed to drop" とあるので、意味は恐らくその表現と同じ意味で「分かる」、「気がつく」ではないかと推測しますが辞書で確認します。まず "twig" の意味は：
・Merriam-Webster's Online Dictionary: 1: notice, observe 2: to understand the meaning of : comprehend: He seemed confused until he twigged that something was going on.; I had to explain it to him three times but he finally twigged.
・Cambridge Dictionaries Online: to suddenly realize something: Then he twigged what I meant; She's six months pregnant and he still hasn't twigged.
次に慣用句らしい "penny seemed to drop" の表現を辞書から探し出します。
("the) penny dropped" あるいは "the penny drops" の表現が辞書に載っていました。
・Collins English Dictionary: informal mainly British: the explanation of something was finally realized
・English slang and colloquialisms used in the United Kingdom: the) penny dropped Noun. Used to signify that a person finally understands something. E.g."Thank God for that! The penny's finally dropped and we can all go home now." {Informal}
・Cambridge Dictionaries Online: If the penny drops, you suddenly understand something: She looked confused for a moment, then suddenly the penny dropped.
英国のスラングの様ですが、おそらく英国の自動販売機はコインが中々すんなりとは落ちなかったのでしょう。昔、米国のホテルで飲料自動販売機が故障しているのに遭遇した事が何回もありました。