Strainers are usually downed trees or a collection of branches and trash. Do whatever you can to avoid these death traps. If you accidentally swim into a strainer, work very hard to climb up on the structure rather than getting impaled or strained by hidden limbs. Here is an example of one on the Savage River in Maryland. This is one place where the defensive swimming technique will fail and you will be swept under. If you can't avoid a strained, swim head first and quickly towards the strainer. When you reach the strainer, push yourself over the branch and kick your feet to help push you over. This video of the LA County life guard training shows how challenging swimming over strainers and why we practice the strainer drill: Strainer Drill.