Light defense comes in two flavors: stay-home LD and chaser LD. A stay-home LD uses a laser rifle and snipes at enemies or cappers leaving with the flag, while a chaser LD will chase the flag carrier out. The distinction is notable because a sniper can't retain their mobility when using a laser rifle, while a chaser may have different weapons like a grenade launcher which can be used for chasing when you see that a capper will end up in an easy position to be hit by it.
The keys to being an effective LD are aim and awareness. You obviously need to be able to hit your targets to be worthwhile, and you need to be able to look around and see anyone coming in. If other enemies distract you, you need to make sure that there are LD still working the flag before you go off and attack them. It's best for a stay-home LD to go off and attack enemies because they can snipe the flag carrier from almost anywhere.
On some maps an LD needs to be able to bodyblock cappers coming in. Removing all of a capper' momentum will ensure that it's very difficult for them to get out with the flag. It's best for a chaser LD to do this, because a sniper won't have the energy to both stay in the air to block an enemy's approach and still be able to snipe.