In a normal forward walk the body is forward over the ball of the foot, ready to continue forward into the next step. Following a checked forward walk however the next step will be backwards - the forward motion is checked.
To allow for this the body, and its weight is not taken as far forward, and the weight is not taken fully onto the front foot. The knee of the back leg is allowed to close towards the back of the front knee also helping to check the forward momentum. The characteristic toe-turnout is used in a checked walk more than in a normal forward walk where it would result in an unnatural action.
Note that in a basic step, when you take a checked forward walk your partner will be taking a backward walk and their body will move slightly further than yours so you will separate slightly.