Can anyone tell me what happens when rolling backwards while pedaling forward on a freecoaster bike? I mean does it gradually slow it down, or does it come to an abrupt hault? I have not tried it yet.
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As soon as the freecoaster engages it will come to an abrupt hault. If you do that while going backwards that means that your front end will most likely want to rise. In BMX this is usually accompanied by a 180 degrees movement of the body so the bike turns and and you continue forward with the momentum you already had going backwards.
Note that a freecoaster may have been set in such a way that it has some degrees of slack (a dead period) between pedaling and the moment the freecoaster engages. This, for example, can be set to 60 degrees (1/6 of full circle) or even more if you are into bmx flatland riding. The reason is that on some tricks you need to be on the pedals going backwards (possibly only on the rear wheel only) so you really wouldn't want an accidental engage of the freecoaster due to your foot minor movements while trying to balance.
I think there is some confusion about the difference between a Freecoaster hub, a Freewheeling hub, and a Coaster brake hub. So briefly:
Above is a hastily drawn sketch of a freecoaster hub. There are no pawls...instead is a cone shaped clutch which is threaded onto the driver body which extends over the axle. The driver cog on the left is supported on the axle by bearings on either side (there are also lots of other bits and pieces, but these are the general parts.) When pedaled forward the driver threads through the clutch and pulls it into the wedged surface of the hub. The friction between the hub and the clutch is what drives you forward. When you change directions (or pedal backwards) the driver threads the clutch to a backstop which adjusts the play from freecoasting to pedaling forward.
If you were to pedal forwards while coasting backwards, your pedal force would engage the clutch into the hub shell. -- As @Daniel R Hicks pointed out, this is pretty much the same mechanism that a coaster brake utilizes except in reverse. Pedaling forward ought to engage the clutch and slow you down. --
However...I'm not sure that I recommend that you do this because neither the clutch surface nor the hub shell are designed for slowing, only for engaging. (If anybody has more info on the actual construction of the hub shell...whether there is a steel insert to bear against and provide braking support, or whether the driver and hub shell are forward motion only, please edit away.)
Here is a fantastic video about freecoaster anatomy.
The short answer: You will probably fall on your back, so it's not a good idea to try doing this on high speed fackies.
The long answer: As answered before me, you stop. That's why most of BMX freecoasters have quite big angle of free crank turn (and it's even adjustiable in some models, like the KHE feecoaster), so when you land in fackie (for example from 180 from stairs) even if you go a bit forwards on the pedals (and it's quite reasonable you'll do it), you still be able to roll back. Otherwise you would be flying on your back (same thing like when you lock your front brake). Note, this way you can't do lots of tricks that are meant to do with tailtap (360 with taps, brakeless tailtap and so on).
Unclear whether the question is about "freewheel" hubs or "coaster brake" hubs.
With a freewheel hub, if you roll backwards, the pedals will necessarily turn backwards. If you "lock" the pedals with your feet it will be impossible to roll backwards.
With the standard coaster brake hub, if you roll backwards and do not exert any force forwards or backwards on the pedals, the pedals will not move and you will roll freely backwards. "Back pedaling" will apply the brakes, and pedaling forward even a little will engage the pedals with the wheel and hence convey the motion of the wheel to the pedals (and vice-versa).
(Though I have to admit that it's been so long since I've used coaster brakes that I may be misremembering some details.)