Quicklocking
A quicklock (QL) is just that, a quick lock.
If you can catch someone off guard and get a QL on them, you'll often get the kill. As a way to practice this, try some matches where all you do is try to QL. Advance through the map while pressing A and hovering your cursor along the edges of your vision as it expands.
If you lock quickly enough, you can stop an NL with a QL!
Example QLs.
You'll find it easier to QL travelling down or right. So, when you have the top base and you're attacking downwards, you will generally have an advantage for QLs.