Cross

The rear straight punch is commonly referred to as the cross. It is more powerful than the jab due to the rotation of the upper body in conjunction with the rear foot pushing off the floor. In terms of speed, it is slower than the jab due to the greater distance between your rear hand and your opponent.

When throwing the basic cross:

-From a left lead, push off and pivot on the rear right foot. Rotate your body to the left and shift your weight onto the front foot. 

-Throw the punch in a straight line out and back.
-Make the punch snap. Keep the hand loose, tightening the fist just before impact.
-Do not chicken wing your arm by rotating it out to early and in an exaggerated way as this causes it to loose power and telegraphs the punch.
-Protect your chin by keeping it down, lifting the shoulder of the punching arm and keeping your opposite hand up.

Fist Positions
Generally the fist is in a horizontal position upon impact. This is the most common way it is taught. Some variations include the verticle fist or the angled up or angled down position. 

Head Position

There are two general head positions you can use when throwing a Cross: Middle,  Lead Side.
1) Middle - Head doesn't shift off line when throwing the cross.
2) Lead Side - Head shifts to the lead side. Often used in conjunction with a lead side angled step forward. 

The cross can also be thrown from a lowered position.

1) Low Middle - Cross thrown from a low duck position.

2) Low Lead Side - Cross thrown from a lead side bob position. 

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