Refreshing Twitter Feed
Here's another option that works with most apps that require you to pull-down to refresh.
These apps will generally be displaying a list of items, so for best results make sure that VoiceOver focus is on the first item in the list.
Now do a one-fingered double-tap, but do not lift your finger from the screen after the second tap (this is often referred to as a double-tap and hold). Whilst keeping your finger pressed to the screen slide it down about 2 centimetres and then lift. Alternatively, you can do a downward flick of your finger after you have done the double-tap and hold.
I would suggest that you experiment with some of the options that have been suggested, and see which one you find easiest and most reliable.