Without some kind of proportional brake (many times called F3A brake), you will nearly always experience that a model with an outrunner accelerates downwards. Setting an idle using the ESC will usually not be enough to stop the motor from spinning up going downwards. If you are using one of the Hacker X-series of controllers I don't think they have proportional braking, but the Spin controllers do, if you have the spin prog box.

With an inrunner using a gearbox, setting a low idle will in some cases work, this is due to the mechanical advantage of the gearbox. When the wind tries to spin the prop it will have to spin the motor with an RPM that is the prop RPM multiplied by the gearbox ratio, meaning that even a small amount of braking on the motor will be more effective. On the outrunner you don't have the mechanical advantage of the gearbox during braking, here the ratio is 1:1.

Hope this makes sense...