Please help me explain what happens after Tom encounters the "sheer nothingness."
"a fraction of his mind he was knew he was going to fall, and he began talking rapid blind steps with no feeling of what he was doing, siding with a clumsy desperate swiftness, fingures scrabbling along the brick, alomost hopelessly resigned ti the sudden backward pull and swift motion outward and down. then his moving left hand slid onto not brick but sheer emptiness, an impossible gap in the face of the wall,and he stumbled. His right foot smashed inti his left anklebone; he staggered sideways, began falling, and the claw of his hand cracked glass and wood, slid down it, and his fingertips were pressed hard on the putty edging of his window. his right hand smacked gropingly beside it as he fell to his knees; and, under the full weight and direct downward pull of his sagging body, the open window dropped shudderingly in its frame till it closed and his wrists struck the sill and were jarred off"
Picture in your mind what is happening here. Tom Benecke is out on the ledge and trying not to fall off. In that sort of case, you would have your front pressed against the wall and you would be moving your hands (pressed against the wall) as you go. You would be expecting the wall to keep feeling the same. Then all of a sudden your hand hits space rather than wall. You lose your balance and various parts of your body smack various parts of the wall as you desperately try not to fall. That is what's going on here.
Tom's hand has gotten to where his window is and so it encountered empty space because the window was sort of set back in from the wall the way windows are. So when this happened, Tom almost fell.