What this does is keeps you as mining when you go into your inventory, it stops the current block you are on, but when you return to your game you won't be swinging, instead you'll be standing there breaking blocks. This is nice, because if you are say clearing a trees leaves that were left behind and you keep miss swinging, it doesn't count the miss swing and immediately goes to mining the block instead of taking that half a second or so. Really, this is a minor time saver but I find it useful if I'm going for speed and want to clear something out nice and fast.Edit: Thought I should mention, if you haven't turned your Inventory key from I to something more local, like E, do it. Another time saver, keeps your hand on the mouse and you can always keep walking.
When mining, take a lot of bones, a couple dirt, a hoe, grass seed, and a workbench. Bam, instant bread.