Hmm, at the very least I think the intent was you choose as 4th level or lower spell and a trigger {as defined by the readied action), that will activate this spell, using it up. (So, only once per day, and the trigger can be the most simplest of things. However, one could argue that the trigger would need to be something outside the users control, as the readied action stated in the feat says "You prepare to use an action that will occur outside your turn. Choose a single action you can use and designate a trigger."-p308, so you would not be able to set the trigger of you doing something on a foes turn, even though you have the trigger set up for the entire day. Still, the work around to this is to have the Trigger be "When some says 'Miki-Miki'" and then have one of you buds say it when you need the spell, if this is the case.)
As for what it actually does, well, while you do prepare it and the effects go off when the trigger is happens, it does not say you casted the spell. So it could very well be interpreted as you still have the spell, as you never technically casted it.
However there is one other thing that needs to be considered, and that is Readied Actions. The two main changes to Readied Action in the feat is 1) the readied action to trigger it lasts the entire day instead of one round and 2) it allows for actions that would normally take up to 10 mins, instead of a single action {though arguably going back, even if a spell normally as the 10mins casting time, your not actually casting the spell, just using the reaction to gain the spells effects). So, does it mean now you have infinite triggers, or once the trigger occurs and the readied action happens, it ends as the readied action as been fulfilled? (Personally I believe the ladder, meaning you only get one, but I could see an agreement going the other way.)
