I've just toyed around with this tap to wake and I've noticed some things about how it works.
You have to tap in the same place (you can't tap in corner and then then the other corner) and the distance seems to have only half an finger width margin of erorr, you have to use a small area to tap, so you cant tap with your whole thumb for example, it works best with just a finger tip. It doesn't matter how hard you tap but timing is important and it's roughly the same speed as a typical double click on a mouse (the faster the double tap the better).
Using this method to wake the phone is pretty redundant I think, I prefer to use the power button.
Anyway hope this helps.
Phone History: -
Ericsson A1018s > Ericsson T20e > Nokia 3510i > Motorola V600 > Motorola V3x > Sony Ericsson K800i > Sony Ericsson Xperia X1 > HTC Desire > HTC One X > Sony Xperia Z2 (Current)