It's right here...
"Sometimes you want to get the jump on a foe or wait for a particular circumstance before you act."
It has to be particular, which means specific.
From the Dictionary:
Circumstance
1. a fact or condition connected with or relevant to an event or action.
In game terms this means that circumstance refers either to an event or to an action. We all know that characters can take four kinds of actions only: "Move, Bonus, Action and Reaction". So the trigger will have to be one of these. The "start" of spell is really the casting of a spell from a bonus action, an action or even a reaction. However you want to call it, the readied action will take place after the spell which is the trigger.
But what about the "event"?
Well the event isn't made by a characters, monsters or NPCs. An event is something that happens outside the control of PC and NPCs alike. It can be as silly as: "I wait for the door to start closing to put my foot in so that it won't close." to "I wait for the geyser to stop pouring out water before jumping." Or even "I wait for a cloud to hide the moon before sneaking out of my hiding place".
So if you respond to something from a character, monster or npc, you then react to one of the four types of actions that the rules allow. Thus you will act after the triggering action. In our case, the bandit will then strike the PC after the PC has been healed.
If you respond to an event, something not done/caused by a PC, NPC or monster then your reaction will be after the event.
So in any case, by RAW and RAI, you will take your reaction after the triggering circumstance, from whatever or whomever it comes from.