True snaffle bits, with rings that attach directly to the corners of the mouthpiece, are "direct pulling" devices. They act on the tongue, lips, and corners of the mouth without altering the degree of force applied to the bit from the rider's hand.
If the rings are large enough and appropriately shaped, or the snaffle has full or half cheek-pieces, most snaffle bits do not need a chip strap. However, when the rings are small, a chip strap or bit stops (the round rubber or leather accessories that fits on either end of the mouthpiece) may help prevent the bit from being pulled into or through the horse's mouth when pressure is applied to one rein.
If you use a chin strap with your snaffle bit, the chip strap should be placed in front of, rather than behind, the rein attachments so as to not interfere with rein pressure on the bit rings.