That would display your starting stack from your previous hand.
amt_before is the stack at the start of the hand that is imported, so after hand A is imported, hand B starts, and amt_before is the starting stack of hand A.
There is no queue; PT3 reads a whole hand once it finishes, and then imports all of that information.
As I think about it, I remember the problem now: the stats on the HUD are all from the Player Statistics section (either HCPS or HTPS), which all deal with groups of hands. Displaying information about just the last hand in the list doesn't really work. You could do it, I suppose, if you used the "only display last X hands" and set X to 1.
Though it might be possible to do something using the date functions but I haven't worked out entirely how to use those to their fullest capacity. (The idea would be to have the stat equal to the sum of amt_before for whatever hand happened in the last 10 seconds, have the stats update automatically every hand, and that might just pull the data from the very last hand, but I think you might need to have the cache disabled in order for it to work right or the date functions won't be calculated properly as the value will be cached at the time of your last cache update . . . but since we're going for a very narrow window you might be able to avoid the cache problem if you wait until a while after the last hand was played before caching . . . basically, I need to test this to be sure either way).
I'll take a look at it though, if that information is grabbable, I'll definitely try to add it to my advanced tourney HUD which I hope to finish sometime within the next couple of weeks (Beta 24's release made this week a bit more hectic than originally expected so I made less progress than I'd hoped . . .).