Scratch

This is to do with rendering speed. In normal view in Scratch changes show more quickly (Possibly because the screen is smaller) so very fast movements or costume changes are more likely to be spotted whilst the online flash player and the fullscreen mode don't spot such small changes. What you want to do is to set your sprite to the sensor costume and perform whatever actions it needs to do as fast as possible before switching back to the main costume you see. (It's a good idea not to use wait blocks or loops whilst the sprite is in sensor mode).

I hope this helps, I'm not quite sure if this is what you mean.