I have seen some people throw seed without using dirt. Is it less or more efficient ?
Here is this video here that shows what I am talking about
In the video, he applies a soil-based topdressing material after spreading seed on the lawn. I'm not sure where you're getting the "without using dirt" from. If you simply throw grass seed onto compacted soil, you will get poor germination.
I'm not a lawn expert, but if I was going to go to the cost and effort of spreading grass seed on my existing lawn, I would definitely spread a thin layer of finely sifted compost or topsoil over the top of the seed. If you have bare spots, I think it would be worth the effort to scratch these up so that the seeds make good soil contact when they land there.
For what its worth, I conducted an experiment to determine the best way to sow grass seed before reseeding my lawn with $200 worth of KBG seed.
In pot #1, I just sprinkled seed on the dirt. Nothing else.
Pot #2, I sprinkled seed and covered with a light covering of dried grass clippings.
Pot #3, I sprinkled seed and then rubbed my hand overtop lightly to similate "raking in".
Of the 3 pots, and to this day, pot #1 is the densest. Pot #2 has the tallest grass, but not as dense as pot #1. Pot #3 had the poorest germination rate and has the shortest grass. Actually, pot #3 is quite pathetic looking.
It seems the worst way to sow KBG is to cover the seed with dirt. This may be different with a variety having larger seed, but KBG seed is very small and seems to lack the strength to emerge from under the dirt... or maybe the seed rotted in the ground. Its hard to be certain.
The dirt used in the pots is the same dirt as my lawn, which was mixed and sifted, then distributed to the 3 pots. The pots were kept in the same location and were watered by rain, so watering was even between the 3. The soil had a loose texture on the surface and was in no way compacted when the seeds were sown.
Unfortunately, conditions prevented me from just throwing the seed on bare dirt, fearing the seed may dry before germination or the rains would wash it away. It takes a full 7 days before KBG seed sprouts. So, my lawn got the pot #2 treatment since pot #2 had an acceptable appearance and seemed to give the best odds for at least a decent germination rate in unpredictable weather.
I have no regrets and mulching seed lightly with grass clippings seems to be an acceptable way to sow grass seed as this most closely replicates nature where seeds fall to the ground in the thatch between grass blades. Grass seeds in nature are not generally covered with soil.