I will try to explain what I understood, I'm not a master... Please try to follow it on the diagram at http://senseis.xmp.net/?Probe (section "Another example")
In many situations (especially when in a corner are only black stones or only white stones - let's call the player in that corner the Defender), the Attacker is, obviously, at a disadvantage. Any line of play he chooses (like invading the side, trying to live in the corner etc), the Defender will have some powerful moves which will have multiple purposes, like defending the corner while pressing the opponent's stones. These multiple-purpose moves exists in ALL lines of play.
Let's call the Attacker's yosu-miru move Y.
Now, after Y, the Attacker can choose a line of play where R doesn't have a multi-purpose value. Maybe it's just a move which defends the corner (without attacking Attacker's stones), or maybe it's just a move which presses the Attacker's stones (without protecting the corner).
More technically, I guess it's a matter of destroying Defender's potential.
Let's assume now the following starting situation:
- if Attacker plays A1, the Defender responds with D1, a double-purpose move
- if Attacker plays A2, the Defender responds with D2, a double-purpose move
Both lines of play are not so good, so the Attacker plays the yosu-miru move Y. Let's assume that the Defender has 2 possible responds, R1 and R2.
- if Defender plays R1, the attacker can play A1, and D1 is no longer a double-purpose move
- if Defender plays R2, the attacker can play A2, and D2 is no longer a double-purpose move
Don't take it too seriously, as I'm not a strong player...
