I have no idea who Flo is but they came up with a really nice trick. Flo's Mess is a very graceful & flowing pattern, when done well your arms sway from side to side as they cross & uncross.
Start with two balls. Hold one in each hand with your arms held out at waist height. Begin with an Under the arm throw with one hand, swing your arm & throw the ball so that it curves in slightly & lands in the middle of your pattern. After releasing the first throw immediately begin to carry the other ball underneath & to the outside of the one in the air. When the under the arm throw peaks throw the second ball as an Over the top throw that follows the path of the first throw into the centre of the pattern. Catch the first throw with the same hand that just made the over the top throw & then catch the second throw in the other hand (the one that threw under the arm). Practise this on both sides.
Do the same again but this time when you catch the first ball carry it down & go straight into the under arm throw on the other side. The second ball is caught with the arms crossed by the hand on top. Practise until you can flow from two throws on one side into two throws on the other & back again, continuously & smoothly.
Continue juggling just two balls in this way & watch what your hands are doing. Notice that while one hand is carrying a ball to the outside underneath the other ball, the hand that made the under arm throw is momentarily empty. During this gap click your fingers. In the full pattern the third ball will fit into this gap.
Now get your third ball & juggle a cascade. Lead into the pattern with an under the arm throw, as you release the ball bring the hand back to catch the third ball. Continue just as you have been practising with two balls with a carry & over the top throw of the second ball before catching the first ball. As the second ball peaks throw another over the top throw over everything to the other side of the pattern, move your throwing hand over to catch the second ball. Make this over the top throw slightly higher than the rest of the pattern to give yourself time & space for the underarm throws & the carries. At the same time as you make the over the top throw you also need to start the scoop of the under the arm throw on the other side of the pattern. Carry underneath that throw & just keep going.
The key to this trick is feeling the flow.