Swash is what occurs after a wave breaks. It is the resulting amount of water that gets sent toward the shore. In a constructive wave, the swash is more powerful than the resulting backwash. This difference enables the shoreline to receive more deposited materials because what is brought in from the ocean is always greater than what is taken away.
Backwash is when water is moving back towards the sea along the beach. Because constructive waves have a very weak backwash, they have little effect on altering what the swash has done in depositing material on the beach. They are not strong enough to carry a lot of material away as they head back towards the ocean, although they do take some material back out to sea. The backwash from constructive waves will largely be absorbed by the sand. The following swashes won't be affected by the backwash as they come in because of the weakness of the backwash.
Constructive waves also have a low amount of energy and a low frequency in reaching the beach because they predominate in much calmer weather than destructive waves. Destructive waves are more powerful due to inclement weather, which infuses more energy into the water through strong winds. Constructive waves only roll in between six and nine waves every minute. Destructive waves come in at about 14 waves every minute. Constructive waves are long and low and do not crash violently against the beach. They roll in calmly to the shore.
Constructive waves have the effect of developing a wide, lightly sloping beach. They also deposit materials from the sea onto the beach to build it up through the particles it transports. They break more lightly than destructive waves and sometimes are not even breaking. They just make their way up the beach with less and less power.