Snaffle bits are the most commonly used bits in ponyplay (and in the bio-equine world). A snaffle acts by direct pressure (i.e. it does not act by leverage). The typical bit gag (a straight rubber bar) used in BDSM and ponyplay is an example of a snaffle. In my opinion, the third most common bit-gag (after the straight rubber bar, and the leather bit) seen in ponyplay is the rubber coated jointed snaffle.
For example in the top pane of the image at the right are three snaffles (the middle one is a standard rubber bit gag, which is a straight mouth rubber snaffle in the bio-horse world). In the bottom pane of the photo (with the red "X" through it) is a gag bit (a gag bit uses leverage though it is not the same as a curb bit): this is not a snaffle. A snaffle merely describes the action of the bit, not the mouthpiece.
Snaffle bits have a multitude of mouthpieces (as can be seen in the photo at the top right), which can make them very severe to the human pony. For example, a simple rubber bit gag with the addition of a juba port can stimulate the gag reflex when the reins are pulled.
Just because they act by direct pressure, doesn't necessarily mean that snaffles are mild. However, a curb bit or a gag bit with an identical mouthpiece will likely be more severe because it puts additional pressure on the chin, poll (top of the head if the human pony has a bridle capable of accomodating a leverage bit) and, depending on the mouthpiece, the roof of the mouth.
An easy way to tell the difference between a snaffle bit and a curb (or even a gag) bit is to look at the number of rings the bit has. A snaffle bit will have only one ring on each side of the bit (the reins attach to same ring that the bridle attaches to). A curb bit or a gag bit will typically have at least two rings on each side of the bit: one ring (the upper ring, usually closest to the mouthpiece) will attach to the pony's bridle, while the second ring (the bottom ring, which, on a curb bit, will be futher from the mouthpiece; on a gag bit it may be of similar distance, but on a gag bit the mouthpiece will not be fixed to the bit rings, but will be able to slide along them) will be where the reins attach.
Now from experience, I have found that jointed mouthpieces are quite ineffective on the human pony. However, they are aesthetically pleasing: I absolutely love the look of a jointed bit on a ponygirl. And just in case you care, here are a bunch more bits that are technically all snaffles: