Davy,
Welcome to the forum!
The manuver is simple, but it takes a little nerve and practice (if you choke, you will crash).
- TURN OFF ANY AUTOLEVEL (you can flip in multiwii's "horizon" mode but for the first few times, you don't want it fighting you as it comes out of self level)
- make sure your rates/stick scaling are high enough to perform the manuver -- the faster you can roll/pitch, the better.
- first, get used to throttle pumps -- get into a stable hover, then punch the throttle to near full for 2-5s, and back off. descend and repeat until you feel comfortable with this
- ***when you're ready to break your quad into a million pieces*** do a throttle pump, and as you reach the top, as you back off throttle, give full hard elevator (or aileron)
-- as it rotates past 90 degrees your throttle should be down to about 1/4 -- **don't go lower!** if your ESCs shut off, you will crash.
-- As it rotates through 270 degrees begin your catch -- pump your throttle up to ~3/4
- as it comes level flatten her out and bring her back to a hover
- Repeat