Hey Elizabeth,

The short lines
does work by giving you probably another 4-5 knots top end (but you lose it on the bottom end. The way it works is this:-
1. Have you ever noticed how much your back lines loop downwind when you slack the rear lines by allowing the bar to full detent against the stop? This is due to drag of the line itself being in the wind held perpendicular to the wind direction. If you remove 10m per line from the kite end of the lines or bring the kite closer to you, then you effectively remove 40m of total line length dragging the kite downwind. What happens when you drag your kite further downwind? It sits deeper in the power window which gives not only more power, but more power from further back in the wind window aka fighting against your upwind efforts.

2. The shorter lines create a smaller wind window which reduces the amount of arc the kite flies through from one side of the window to the other. So working on angles if the kite sits 10 degrees further back in the window when you are traveling fast, and as you apply brakes or resistance to slow the board thus forcing the kite to the neutral at the edge of the window, with 30m lines, it might be say 6 metres from in the power when riding forward to neutral. On 20 metre lines it is only 4 metres, and as the kite is more responsive due to less line flying through the air, you are much more easily able to edge the kite to the neutral as it travels the distance 33% faster and has 33% less distance to travel, it can be more than twice as easy to accomplish control. Think about it!
3. It allows you to ride upwind more easily as the kite, on it's own, sits higher in the wind window which means it is pulling you from an angle closer to the relative wind. The kite is also more responsive and "crisp" to steering because you have 33% less stretch effect over the 20m lines. As your lines are short so shall your wind window be which means it is incredibly fast to get the kite from one side of the wind window to the other. (great for wave riding down the line).

4. Kite loops are unbelievably fun, it is easy to get above your kite and get a solid whip. All good reasons why 20m lines rock!

Now the downside:-
1. It makes the kite much more lively in its reaction to gusts which can create a real jerky "on and off" feel to the kite when encountering gusty conditions, especially really high A/R kites.

2. Jump timing, height of jumps and lofting time go out the window. It can be done, but it aint as easy as 30m lines which give a large margin for error in timing.

3. Kiteloops are much more scary and when they go wrong they can rally go wrong (prepare for a beatin')

4. It takes time to get used to the feel of shorter lines and the kite will catch you off guard repeatedly until you catch up to it as it will be where it should be much quicker.

All in all, it is another part of kiting that you should explore to see if you like it. It can be cheaper than another smaller kite if you are prepared for the trade offs (personally the trade offs suit my riding style). I use my 9 Reactor with two bars (20m and 30 m)and two boards (6'2" with straps Converse Surfboard and a 145 Mutant for when its cranking) and and it has a wind range of 10 knots to around 30 knots.
Try for yourself and see!

Good winds to all and thanks for reading this long post if you took the time