between her teeth

Who's afraid of Virginia Woolf?は戯曲なので状況や登場人物の動作、表情、考えを[ ]内に示しています。
今日取り上げるっ表現はその中にあったものです。
[The front door-bell chimes.]
MARTHA: Party! Party!
GEORGE: [murderously]: I'm really looking forward to this, Martha...
MARTHA: [same]: Go answer the door.
GEORGE: [not moving]: You answer it.
MARTHA: Get to that door, you.
[He does not move.]
I'll fix you, you...
GEORGE: [fake-spits]: ...to you....
[Door chime again.]
MARTHA: [shouting ... to the door]: C'MON IN! [To GEORGE, between her teeth] I said, get over there!
GEORGE: [moves a little towards the door, smiling slightly]
"between her teeth" が今日調べる表現ですが、これは慣用句らしいので辞書で直ぐに見つかりそうだと思ったのですが、OneLookの検索では "between one's teeth" の表現の項目は見つかりません。しかし、"get/take the bit between your teeth" の慣用句として次ぎの様な説明が見つかりました。
・Cambridge Idioms Dictionary: (British, American & Australian) also take the bit in your teeth (American): to start doing something in a very keen way When the team really gets the bit between their teeth, they are almost impossible to beat. (British, American & Australian)
・McGraw-Hill Dictionary of American Idioms and Phrasal Verbs: to put oneself in charge; to take charge. Someone needed to direct the project, so I took the bit in my teeth. If you want to get something done, you've got to take the bit between your teeth and get to work.
・Dictionary.com: take the bit in / between one's teeth, to cast off control; willfully go one's own way: He took the bit in his teeth and acted against his parents' wishes.
・The Phrase Finder: Take control of a situation: Origin: A bit is a mouthpiece that is used to control a horse's movements. It is normally fitted so that pressure on the reins presses the bit against the soft parts of the horse's mouth, causing it to turn its head. This expression alludes to a horse biting on the bit and taking control away from the rider.
"bit" が馬具のはみを意味しているとは意外でした。
"between her teeth" は小さい声で静かに言うことかとも思ったのですが、ドアに行きなさいと言っても動かないGEORGEに対し、MARTHAはきっぱりと命令していると解釈すれば良いのでしょうか？
そこでしつこく調べるとReal Dictionaryに "Synonyms for between the teeth (adverb): sotto voce, under one's breath" と言う説明を見つけました。 確認の為に "sotto voce" を別の同意語辞書、Thesaurus.comで見ると：
sotto voce: in an undertone: barely audible, between the teeth, in a whisper, in soft tones, under one's breath, with bated breath
とあり "between the teeth" は "barely audible" の意味と解釈する方が無難な様です。 しかし、"take the bit in / between one's teeth" の慣用句の意味も覚えたい。