The Flysurfer Viron is the best trainer kite ever made. It comes in 2.5, 4 and 6m. My school uses them exclusively. They are full four line trainers with pull-pull power strap. The Viron flies exceptionally well.
The true test of a trainer kite is how well it flies to the edge of the window. The Viron does this almost as well as a full size kite. Most other trainers do not fly very well at the edge of the window. Flying slowly through the entire edge of the window is one of our more important flying drills. Flying it and controlling the kite just above the water is another. Most trainers simply can't do this.
The bar of the Viron is of better quality than most full size kite bars. The quality and build is on par with any of the premium brand bars. All the lines, the materials is top notch.
The control bar works exactly as a full size kite. Relaunch is a little like a tube kite whereby you pull one of the back lines. However re-launching this kite is too easy and hence not so good for teaching. Usually when the student crashes the kite, we usually just tell them to wait for the kite to sort itself out. This is also now the case with the latest tubes.
The Viron is so good, we even use it to teach body dragging. And we start lessons right in the water as the Viron is really a mini foil kite.
Other trainer kites
1) HQ Hydra - Another good flyer but a little too powerful if the winds pick up
2) Ocean Rodeo Rise 2m - Doesn't relaunch well but is extremely fast and powerful. Really fun to fly. A great kite to for both students and experienced kiters.
3) 2 line foils - are really outdated, should't even be used for instruction any more
4) Land based traction kites - too powerful and you don't want them in the water
Get a Viron 2.5m for high wind places. For winds 10 to 15knots the 6m is better. the 4m is quite in between.
I highly recommend instructors to get the Viron. Its cost 5 to 10X as much as other trainers, but after students get off the Viron, they are so much more prepared to handle a full size kite. This means they crash your full size kites a lot less. The videos showing the Virons smashing into the water at full speed, well I see that all the time with my Virons.... no problem.
Cheers