Paraphrasing GM Gallagher on the proposed move 19 e4, <"Every time you push a pawn it weakens squares. This time it is f3 and the punishment is severe">.

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Here Smyslov played an -incredible- blunder for a player of his class. He played 18 c3-c1?.

Position after 18 c3-c1?:

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Question: what is the tactical problem with this move? Answer: it greatly restricts the <MOBILITY> of the White b1-queen!

Whenever you move one of your pieces or pawns, you need to make sure that none of your pieces get <TRAPPED> or have their <MOBILITY> greatly reduced as a result. So you need to ask yourself not one question but -two- questions whenever you move:

1) Did I just <TRAP> the piece that I moved? (Escpecially(!) if you just moved a piece forward from a square it cannot go back to.)

2) Did I just <TRAP> not the piece that I moved but a different one of my pieces by <SELF-BLOCKING> a critical flight square or <BLOCKING> a line of retreat?

That is, you must be tactically alert enough to be on the lookout for -any- of your pieces losing a great deal of their <MOBILITY> and thus being on the verge of being <TRAPPED>.

Now we clearly see the problem with Smyslov's 18 c3-c1?. This move -greatly- reduces the mobility of the White b1-queen. In fact, the White b1-queen can move safely -only- along the b1-h7 diagonal, to the d3- and e4-squares in particular! Thus after 18 c3-c1?, the White b1-queen is safe on only the three squares b1, d3, and e4.

So Black (Gligoric) plays 18 ... d7-f5!, taking away all of these squares along the b1-h7 diagonal from the White b1-queen, and suddenly Her Majesty has no where to run away from the Black f5-bishop.

Position after 18 ... d7-f5!:

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How can Smyslov now meet the threat to his queen? The obvious 19 e2-e4 weakens the f3-square and loses a piece to the <DEFLECTION> combination 19 ... f5xh3 20 g2xh3 <deflection from f3> d4-f3+ and 21 ... f3xd2.