My understanding (ive only everhad a couple of vtec and dont profess to be an expert) is this

On some engines its done by tensioning the belt different to retard the timing, on a honda though the vtec cam is actually mechanically altered via oil pressure locking onto a different cam lobe effectively, which means you can have variable timing and lift, not just the timing.

You can also get different duration as well as different timing by this method.