Xray Monkey Jump

Black to play  

In this diagram, black would like to play the monkey jump at a, but it looks like the marked white stone is in the way. However, black a still works, because white is short of liberties. I call this the x-ray monkey jump, because black can see through an apparently solid obstacle. (The clamp at b is always an option in this position, but a is usually the better move if it works.)

White's failure, simple version  

If white simply blocks at , then black can capture some stones this way.

White's failure, more reading required  

This choice of is more complicated, but after , if white tries to block with then again black can capture some stones. Note that at a also works, but this choice of is usually better. If white plays elsewhere at this point, then later threatens to rescue the stones as a ko threat, then this choice of minimises white's possible gain, while black a instead makes for a bigger ko threat.

Usual best play  

Usually, after , white should give way at .

Can't monkey jump when there's a liberty?  

When white has an extra liberty at or near a, or when the marked black stone is missing, usually this means that the x-ray monkey jump fails: white can block safely. But depending on the nearby shape, there is potential for ko: see the next two diagrams.

Ko example 1  

In this position, black x-rays in from the marked stone on the right. Because of the liberty at a, can't connect out, but black can make a ko for the life of the white group.

Ko example 2  

Here, black a next will make a ko.