what is going on here?Q: What is TopoJSON?
A: Check out the TopoJSON Wiki
A: You can supply a comma separated list of attributes manually in the URL (?attributes=foo,bar) if you just want those. Note that you're in trouble if your only attribute column of interest is "true", since that word gets you all of them. Also, there's currently a bug which requires you to add "&delete=true" so you don't get cached results.
Q: Multiple Shapefiles?
A: Yes. Just put them all in a single zip. You'll get multiple GeoJSON outputs and a single (merged) TopoJSON.
A: TopoJSON tends to be smaller than GeoJSON, but not [entirely] by magic. It also generalizes your geoemtries by forcing vertices into a grid; the bigger the grid, the less generalized your geometries (and the larger the file size). Preciesion here refers to the quantization parameter for TopoJSON.
Still confused? See this page for a more coherent answer.