What happens to eyeball when you push your eye muscles harder?
Do you mean squinting? Any sort of tension on the eyes would result in changing of the shape of the eye. A change in shape will bend light differently. The perceived effects you're describing are most likely from you changing the shape of your eyeball to take in more light.
In terms of squinting, eye strain like that won't lead to blindness or worse vision. There's more danger with UV from the sunlight (cataracts) or getting wrinkles from squinting too much in the long-run.