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Antoine,
Two bytecode examples.
For the following function:
>>> def f(x):
... try:
... return x
... except:
... pass
Without the patch:
 2 0 SETUP_EXCEPT 8 (to 11) 
 3 3 LOAD_FAST 0 (x) 
 6 RETURN_VALUE 
 7 POP_BLOCK 
 8 JUMP_FORWARD 8 (to 19) 
 4 >> 11 POP_TOP 
 12 POP_TOP 
 13 POP_TOP 
 5 14 POP_EXCEPT 
 15 JUMP_FORWARD 1 (to 19) 
 18 END_FINALLY 
 >> 19 LOAD_CONST 0 (None) 
 22 RETURN_VALUE 
With the patch:
 2 0 SETUP_EXCEPT 9 (to 12) 
 3 3 LOAD_FAST 0 (x) 
 6 POP_BLOCK 
 7 RETURN_VALUE 
 8 POP_BLOCK 
 9 JUMP_FORWARD 8 (to 20) 
 4 >> 12 POP_TOP 
 13 POP_TOP 
 14 POP_TOP 
 5 15 POP_EXCEPT 
 16 JUMP_FORWARD 1 (to 20) 
 19 RERAISE 
 >> 20 LOAD_CONST 0 (None) 
 23 RETURN_VALUE 
The difference is the 'POP_BLOCK' inserted at offset 7 *before* the 'RETURN_VALUE', so that the RETURN_VALUE bytecode does not have to do any unwinding of the block stack.
For loops the SETUP_LOOP and the matching POP_BLOCK are eliminated:
>>> def f(x):
... for y in x:
... g(x)
Without the patch:
 2 0 SETUP_LOOP 24 (to 27) 
 3 LOAD_FAST 0 (x) 
 6 GET_ITER 
 >> 7 FOR_ITER 16 (to 26) 
 10 STORE_FAST 1 (y) 
 3 13 LOAD_GLOBAL 0 (g) 
 16 LOAD_FAST 1 (y) 
 19 CALL_FUNCTION 1 (1 positional, 0 keyword pair) 
 22 POP_TOP 
 23 JUMP_ABSOLUTE 7 
 >> 26 POP_BLOCK 
 >> 27 LOAD_CONST 0 (None) 
 30 RETURN_VALUE 
With the patch:
 2 0 LOAD_FAST 0 (x) 
 3 GET_ITER 
 >> 4 FOR_ITER 16 (to 23) 
 7 STORE_FAST 1 (y) 
 3 10 LOAD_GLOBAL 0 (g) 
 13 LOAD_FAST 0 (x) 
 16 CALL_FUNCTION 1 (1 positional, 0 keyword pair) 
 19 POP_TOP 
 20 END_ITER 4 
 >> 23 LOAD_CONST 0 (None) 
 26 RETURN_VALUE 
END_ITER is a synonym for JUMP_ABSOLUTE. It is needed so that frame.set_lineno() can identify loop blocks.