What squares are no longer enough defended?
A square might be defended by one or two pieces, but that's not always enough.
The position after 13.f4-f5
Srdjan Zakić - Srdjan Cvetković, Aosta 1989
After 13...exf5 white has no plans to automatically recapture the f5-pawn, since it isn't enough for black that only the king protects the f7-square.
Black played 13...exf5?
...and white replied with 14.Qxf7+!!
Black is lost.
A square might be defended by one or two pieces, but that's not always enough.
The position after 13.f4-f5
Srdjan Zakić - Srdjan Cvetković, Aosta 1989
Black should have played 13...Nde5. There is no time to wait with playing Nde5 and instead playing first 13...exf5, with the idea 14.Nxf5 Nde5.
After 13...exf5 white has no plans to automatically recapture the f5-pawn, since it isn't enough for black that only the king protects the f7-square.
Black played 13...exf5?
...and white replied with 14.Qxf7+!!
Black is lost.