Rasgueo thumb position and technique
This position will give you the best striking attack for the rasgueo, the finger strikes the strings rather than just brushing the surface of them.
The thumb is tucked in behind the fourth string and the index finger is curled up against the thumb, note that the thumb should not put any tension on the string it is merely tucked in, as shown in the photo Thumb Position 1. The C, E and A strings will all be struck and the G string will be muted giving a percussive effect to the rasgueo.
Using the thumb to build up kinetic energy, it flicks the index finger out. The movement is a definite flick not just a straightening of the fingers.
The finger follows through and should finish parallel to the body of the ukulele, as shown in the photo.
After the flick the finger curls back up striking the strings as it travels towards the thumb. Don't worry about how many strings you strike on the way back, it adds variety to your playing.