The design TSR is the ratio of rpm to wind speed that a propeller will give maximum efficiency ( CP ). If you load it too much it will become less efficient as it starts to stall. If not fully loaded it will run faster, there is a reasonable band where it will work quite well, but as you get well towards stall its output will fall very badly.
Similarly with reduced load the output falls slowly at first and with no load it will run away to such a speed that its power balances the losses. At no mechanical load these losses are mainly the drag component of the blades.
The degree of overspeed depends a lot on the TSR and blade profile.It may run away to several times the design speed. Usually the higher the TSR and the more aerodynamic the blades are the greater the overspeed. As a rough guide it is quite likely that a blade designed for TSR 7 will at least run to TSR 12 or even more on no load.
If you don't want your blades to run faster than 400rpm, you must furl when your generator load allows the blades to reach this speed. Rember that furling doesn't limit things sharply at the operating point and also if you loose the load on a machine designed to furl at 400rpm there could still be a serious overspeed as the thrust on which the furling depends is a function of the power extracted.
Try to make sure your blades can survive well over the rated speed to leave a margin of safety. High TSR blades at runaway will be very noisy and start to make helicopter noises.
http://www.windmission.dk/workshop/BasicBladeDesign/bladedesignleft.html#anchor292796
"Quoth the Raven"
