How does Tom succeed in getting back into his apartment?Jack Finney's "Contents of the Dead Man's Pocket"

It is Tom's exactitude which allows him to reenter the
window of his eleventh floor apartment after rashly stepping out on the building's ledge
in order to retrieve the yellow sheet of paper on which he has made times-taking
calculations.  For, he knows that he has but one chance: He calculates against bringing
his arm over his head, as a blow from such a position would not have enough force, and
it would be so awkward that he would surely fall. Tom realizes that he must drive a blow
from his shoulder, but he does not know if his fist will be able to break the glass. 
So, he pauses, contemplating his plan of action.  Then, he knows that he must act;
thinking of his beloved wife Clare, he draws back just a little with his fist so tight
that his fingers ache.  With all his power, he burst through the window thinking of
Claire:

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...with every last scrap of strength he could
bring to bear, he shot his arm forward toward the glass, and he said,
"Clare!"

He heard the sound, felt the
blow, felt himself falling forward, and his hand closed on the living-room curtains, the
shards and fragments of glass showering onto the
floor.

As Tom breaks off the
remaining wedges of glass from the window frame, he enters the room, "grinning in
triumph."