The single jointed snaffle is probably one of the most commonly used snaffle bits. The jointed action allows the rider to put pressure on one side of the mouth more than the other, and hence have better control over the lateral flexion of the horse. 

However, with the single joint snaffle, there is a certain amount of nutcracker action that can occur with greater pressure on the reins. This pinches the tongue, and can cause the horse to evade the pressure of the bit.

Contrary to popluar belief, the single joint is not always the mildest bit and should be used by riders with more experinced and refined hands.