The name comes from a lever a train driver has to keep holding. Let go — for any reason at all — and the brakes go on by themselves. Nobody has to decide anything.
Most reminders ask you to act. This one asks you not to. When you create a schedule, the next message is booked immediately: a real row, with a real time on it. From that moment the message is going out. Your attention is not what makes it happen — your attention is the only thing that can stop it.
That inversion is the whole point. The morning you sleep through your alarm is exactly the morning you cannot be relied upon to send “can't make standup”. So the message is already waiting, and sleeping through it is what delivers it.
Pending is booked and quiet. At your warning time it becomes notified and your phone buzzes. Tap that notification and you get one screen with one button. Press it and the message is stood down. Ignore it and it is sent, from your account, at the minute you chose.
Unless posts the message itself when the time comes, rather than handing it to Slack in advance. So standing down is immediate and total right up to the minute it fires — there is no window near the end where the cancel button stops working, and nothing sitting in a queue you'd have to go and retract.
The trade: if Unless is down at the moment your message is due, the message does not go out. A dead man's switch that fails silent, not a guarantee of delivery.
On iPhone, a web app can only send you notifications once it has been added to the Home Screen. Open Unless in Safari, share, Add to Home Screen, then open it from there and turn warnings on. Without that, schedules still send — you just won't get the chance to stop them.
You. Unless asks for a user token, not a bot token, so the message arrives in the DM you already have with that person and reads as though you typed it. Your token is encrypted before it is stored.