What's the diff between post mount and IS for caliper mounting?
IS: bolts go sideways into a thread on the caliper (or more usually, an adapter)
Post Mount: Bolts go towards the front of the bike, usually screwing into a thread in a stub on the fork.
It's cool that the Lefty is IS because it allows you to remove the front wheel without affecting the side to side position-setting of the caliper. Much better. The IS mounts on the Lefty are slotted so you only have to undo the bolts a couple of turns to remove the caliper / wheel.
Also if you strip the thread in post mount (by overtightening) then you're stuffed (because it's cast into the fork lower, which is usually magnesium). If you do that in IS you just pay £10 for a new adapter.