Hi,
here is just some more explaination about "collision detection" in case you have hardly read those lines.

Now here is simplest example in which network packet contains only one byte. Let say, by some occasion, two devices have the same number. It happens, that the first device sends a byte 11011111b and the second device sends 11000000b. The idea is that the each bit is represented with two bit-time slots. At first bit-time slot, device sets output line to bit value. At second slot, device checks is that level kept. For our example:

      0  1.  1.  0.  1!  0_  0_  0_  0_  0
No1   S--HW--HW--LW--HW--HW--HW--HW--HW--VW
No2   S--HW--HW--LW--LW--LW--LW--LW--LW--VW
      0  1.  1.  0.  0.  0.  0.  0.  0.  1 
bits: 0 1AB 2AB 3AB 4AB 5AB 6AB 7AB 8AB 9AB

Legend:
S - start bit;
L - low level;
H - high level;
W - "watch bit time";
dot - test passed;
! - test failed.
V - validation slot.

To see it simple, assume that at stages 'A' devices just set or clear a port pin. At stages 'B' devices do check that port pin is still at the same state as it was set.

So, as you may see, at bit-time slot 4B device No1 detects network collision because it wrote '1' at stage 4A but read back '0' at stage 4B. As result, device No1 stops data transmit, sets pin to zero (to make collision be pre-detected by other duplicated station(s) as well) and waits till validation slot during which device No1 keeps pin at low state.
At same time, device No2 sends rest of its bits. It does not pre-detect network collision because all these bits are zero. At validation bit-time slot 9A, it sets pin high (says: "I have not detected a transmit error, did someone do?") and reads its state back at time 9B. Oops, it sees the low level, produced with device No1 - we are in trouble then.
For other network devices, if Validation bit-time slot has low level value then it means "drop this packet due there was a network problem"

Hope it is more clean now.
Regards,
Oleg