'Listen, boys. Don't forget now, we want Spanky to win!'
Aside from Spanky's Mom's actual "dis-dressing" onstage, this is my second favorite moment of BL (the first being the moment I wrote about in the post And Her Goose Is Cooked).
As others have mentioned, the moment when Spanky takes his Mom aside backstage to tell her that he needs to "tell the gang something" is where we see her the proudest. She actually - for the first time - is smiling with genuine happiness. In fact she's practically beaming with pride that her son is showing enthusiasm to perform and win! She even kneels in front of Spanky to listen what he has to say for once. The gesture is meant to show she is putting herself on his level, you might say. It really is the only honest reveal of her at her most sympathetic and emotionally vunerable. But of course, it's too late. Even as we might even start to like Spanky's Mom in that moment, we know that she still isn't quite "getting it" regarding the true meaning of the meaning of Spanky's message.
It's no mistake that in that backstage moment, camera angle-wise Kitty Kelly is given her only close-up. She's confident, smiling, her eyes sparkling with enthusiasm. I dare anyone to not find her beautiful, even sexy, in that moment. Throughout the entire short, although while still beautiful, Spanky's Mom is shown in mostly from medium to wider angles and in a "motherly" (and a pushy one at that!) way. In the way Kitty Kelly is shown confiding with Spanky, it is almost a subliminal suggestion to see Spanky's Mom as, let's face it... kinda hot.
To prepare the viewer for Spanky's Mom (some say, "unjust" or "too far") embarrassing exposure, the next scene goes even further to support with subtlety we see Kitty Kelly as "sexy" or a "MILF" (to use a current phrase). As Spanky's Mom exits the side of the stage and down the stairs to the front row, it's the first time her coat opens to really give everyone a good look at just how tight her dress is, how the material clings to a curvy figure, the buttons drawing our eyes either up to her neckline or down to her legs (depending on if you're a cleavage or leg fan). And when she leans just slightly over the boys with a smile? Her pose is just slighly - almost cluelessly on Spanky's Mom's behalf - sexy and there can't be doubt this was not intended intentionally by the director and/or writer.
But... just when we're beginning to like her, that "pushiness" of Spanky's Mom appears in the way she talks down to the Rascals, almost ordering them around, using her status (and maybe a little of her looks) as a parent, as an authority figure to tell the gang what to do. It makes us see her as the controlling and snobby know-it-all that we all want to see get a comeuppance. When one of the gang insists, "Don't listen to what his ma says!" its the invitation to us to get on board with the gang... but I'm sure neither the gang or Spanky's Mom or Spanky really could have thought that their combined antics would result in Spanky's Mom having her dress ripped off onstage leaving her helplessy dazed in her lingerie in front of a laughing, cheering, howling audience of her friends and peers.
Which would all lead to the obvious: the ones who derived the most joy, pleasure, and excitement out of seeing Spanky's Mom's dress torn off in such hilarious and humiliating style would be Spanky's friends in the front row. Based on her attitude toward the gang, it couldn't have been the first time Spanky's Mom acted so pushy to Spanky's Friends. It's proven by the way they react when she approaches them in their seats. The gang manages to hide their props in time, but none of them greet Spanky's Mom with friendliness. They're guarded and uncomfortable (for obvious reasons) but they also don't show any connection to her. And once again when we hear "Don't listen to what his ma says!" that shows the gang doesn't really like or have any respect for Spanky's Mom. So to see their friend's "MILF" get her comeuppance onstage leaving her exposed physically and emotionally (the dizzy ditzy submissive expression on her face as she looks up at her ruined dress dangling from the curtain above her) you can bet it's the Rascals you can hear laughing the most.