Based on the experiments with English and Japanese that are referred to in the textbook, we would expect that in the case of the English sentence
a. The bottle floated into the cave
the speaker might perform a waving action with their hand (to show the bottle floating) while at the same time moving their hand in a direction to show movement into the cave. The Spanish speaker saying
b. La botella entró a la cueva flotando
might on the other hand produce an action showing more plainly a directional movement and might only separately - if at all - produce an action intended to show the floating or bobbing of the bottle.