time step works like this:
the computer calculates the sim as if everything is frozen and only one object moves
go on to the next object
repeat
when all the objects positions have been calculated, they are all moved to the updated position and the whole thing repeats.
if you turn your time step up, the computer calculates the objects farther and farther in time apart, resulting in small errors that accumulate over time. If your orbit is eccentric or unstable, it'll change and or fall apart.