You grab his blocking arm in order to Lop Sao and, much to your amazement, both of your arms are Trapped and he is pounding a hole through your chest. Look again at Figure 3. The real issue is not where the Pak hand is, but where it is going. If your opponent's Pak Sao is going across your center, then you can continue it's motion and do a Lop Sao. If your opponent's Pak Sao is going toward your center and you attempt a Lop Sao, then you end up Trapping yourself.
Defender wins by trapping attacker's hands and punching.
By the time you think of what to do, the energy has changed and you are dealing with a new situation. And in the same sense, timing is critical. You must continue with the correct response before the energy or positions change. You know very well that if you stopped someone's punch, you would not wait around for him to counter before continuing your counter. But I'll tell you a little secret.