An appeal to force is an attempt to persuade using threats. Its Latin name, “argumentum ad baculum”, literally means “argument with a cudgel”. Disbelief, such arguments go, will be met with sanctions, perhaps physical abuse; therefore, you’d better believe.
Appeals to force are thus a particularly cynical type of appeal to consequences, where the unpleasant consequences of disbelief are deliberately inflicted by the arguer.
Of course, the mere fact that disbelief will be met with sanctions is only a pragmatic justification of belief; it is not evidence that the resultant belief will be true. Appeals to force are therefore fallacious.
Example
(1) If you don’t accept that the Sun orbits the Earth, rather than the other way around, then you’ll be excommunicated from the Church.
Therefore:
(2) The Sun orbits the Earth, rather than the other way around.
This argument, if it can properly be called an argument, makes no attempt to provide evidence for its conclusion; whether or not you’ll be excommunicated for disbelieving the geocentric model has no bearing on whether the geocentric model is true. The argument therefore commits the appeal to force fallacy.