"Did you respond to our _____ argument?""Ummm, I believe my partner will be addressing that in her next speech."
To "throw someone under the bus" is a phrase that is rarely taken literally and generally refers to an act of "[sacrificing] another person (often a friend or ally), who is usually not deserving of such treatment, out of malice or for personal gain." To "throw someone under the argument bus" is essentially to do the same thing, but to your debate partner in-round.
It may not be out of malice. It may not be for personal gain. It may be that your partner just forgot to address the Aff's Topicality responses, but since it's the Neg block, throwing you under the bus is apparently warranted and acceptable. But it's going to feel like it's out of malice and for personal gain when you're staring at your flowpad wondering how the heck you're going to be able to fit that into your next speech. Thanks, partner.
It doesn't always have to be for forgotten arguments, though. Often someone just hits a tough question in CX and doesn't know how to answer, so they chuck the responsibility onto their partner, reminiscent of a particularly violent game of Hot Potato.
"Now, you claim our Disadvantage of 'Every American Citizen Goes Broke and Becomes Homeless and Dies In The Street' doesn't stand because they'll get more money from your plan, correct?"
"Where is that money coming from?"
"Uhh... My partner will be getting back to you on that in a later speech."
Now the partner has the hot potato. But he probably doesn't want it either. He'll either take one for the team and respond to the argument like the good debate partner he never had, or assume that since the point was raised in Cross-Ex and few people flow Cross-Ex, it's not a real argument so you can let it go. If things turn out ok, the second guy wins. If things go bad, I would just blame the partner who threw the other guy under the bus. I would never do something like that. Except sometimes. And I would never not fulfill my burden when I am informed that I will be responding to something later. Unless I don't want to respond to it. Especially if it's an argument about the criteria or something. Who cares? Not the judge. Let it go.
Don't be a jerk partner. Unless you have to be. Or you have a partner who just loves it when you tell him what to argue in your next speech when you realize you didn't or can't respond in Cross-Ex. Then, by all means, sacrifice your partner.
You're homeschooled. Watch out for buses.