This term refers to the basic techniques used by the Infantry marine or soldier when he encounters enemy troops. There are three basic movements that I will discuss here but keep in mind that I was an Artilleryman so my explanation may not be the best.
The low crawl is used when moving into an attack position hopefully without being seen or heard. The infantry soldier crawls slowly on his belly to perform this move. He should remove any small twigs before he crawls over them to avoid making noise.
The high crawl is used when the infantry soldier is moving through high grass. The high crawl is done on hands and knees while keeping as close to cover as possible.
Three second rushes are used when trying to close quickly on an enemy position while avoiding being shot. The combatant rushes forward for a three count and falls forward using his rifle to break his fall. He should then roll or crawl to a different location before popping up again just in case the enemy has drawn a bead on his last position. All of my custom military ring customers who are infantry are well versed in these movement techniques. These movement techniques are just a small part of the proficiency training of the infantry soldier.