It's not a mod or hack. You can change it in the controls option. It's called "Pick Block". What you do, you click the middle mouse button (The default assigned) to pickup the block you are staring at. To drop a block, you press Q, which is the default assigned. I hope this answered your questions.
its not a hack just shift+click
I use Bandicam: http://www.bandicam.com/product/how-to-record-skyrim-gameplay/
it shouldn't...it does depend on your PC specs