BR is right -- it's how they will attempt to keep that handful of Senators from getting nuked in the midterms (the ones that had the really really tight election races).
Still, it's a heckuva gamble. Reid will have to be pretty comfortable believing that the House will pass whatever the Senate passes, else he will have exposed those Senators for nothing, because even if they say they didn't vote for Amendment X, their record will still show a vote for the AWB in general, and their opponents back home will surely know of this game, and educate the public most vigorously.
I don't remember the exact number of close-race Dems up in next year, but let's say it's 5. 5 close races, and that was before this whole gun-grabbing thing came up. How are all of you going to vote in the midterms? I'll bet a lot more people are going to express some anger at anyone who voted for the AWB, regardless of what the amendment game says.
If the Dems lose those seats and the Repubs win them, Reid will have handed control of the Senate to the opposition (and if the House doesn't pass the AWB, he will have given it to them for nothing).
You can bet the midnight oil is being burned on this one. This is why it's so critical to contact your reps, even if you're like me, and your rep is Feinswine. The interns who tally the pro / con constituent feedback reports don't just give those numbers to their Senator, they give those numbers to the National Committee. That means Reid will know if there is widespread dislike of the AWB, even if Feinswine doesn't give a crap. That will affect the political calculus.
Reid does not want a repeat of '94, even if Obama wants this one. That's why Obama and Biden sent out those emails a coupla days ago -- they need to drum up the support for these measures, to influence Reid and those jittery politicians on the fence or in tight races.
Contacting your reps absolutely matters on this one, regardless of who it is. Email / call / fax / whatever -- just do it often!