Three times, the user guide refers to atti mode.
But there is no explanation of what that is.
By now, I have guessed what it is, but still have a few questions:

Does the rth button work when the spark has chosen atti mode?
        What does it do then?

Is atti mode the same as sport mode, but without compass and gps?
        Or is it different? How?


How do you suggest I get prepared for the sudden speed increase if
vps mode fails indoor?

What should I do when the go app says "Please switch to atti mode"?

Heliman, RTH will work when your drone has good GPS signal. Without GPS signal, drones will find it hard to position well and fly stably. It is different from sport mode. Sport mode will have a better flight performance. I would recommend you mount props guard when your drone flies indoor so that it can protect itself from being damage.

Please forward my questions to somebody why will answer them.