This is definitely not a normal technique, but can be used when landing on a longish runway and having a high sink rate during the flare due to a loss of energy over threshold (caused by all sorts of different factors, but commonly a loss of headwind). I find a small well timed increase in thrust can reduce the rate of descent on the B737 NG. The under slung CFM 56's produce quite a large thrust / pitch coupling, so this small increase in thrust does two things to reduce the ROD. 1) Replaces lost energy 2) Small increase in pitch attitude.
There is scope for things to go wrong. Too much added thrust, badly timed and for no reason, can lead to a balloon and float effect... not ideal at all!