Occlusion training (aka blood flow restriction training) is one of the easiest and safest ways to rapidly increase muscle size and strength.
Occlusion training is ideal for injured, fatigued or elderly people because very light weight can be used while still achieving a heavy stimulus to the muscle.
Everyone should do this at least occasionally.
Explain like I'm Five
Occlusion training is working out with tourniquets on your extremities (arms or legs). That's pretty much it. I use red Voodoo floss bands, they're inexpensive on amazon. Get them, they happen to be excellent for mobility as well, shown here.
Restricting the blood flow return from a limb traps metabolites (lactate and hydrogen ions) within the muscle tissue. Metabolites are waste by-products from muscular work.
The blood flow restriction also creates an oxygen debt. The muscles are forced to work in an oxygen poor environment.
The restriction also causes intense pooling and saturation of blood within the muscle, increasing capillary perfusion. It's the biggest pump you've ever had!
All this simply means an icreased stress on the muscle. Stress leads to adaptation. In this case the adaptation is strength and size gains.
An alternate hypothesis: Some researchers believe the reason occlusion training works is due to an increase in muscular activation. Restricting blood, oxygen and metabolites makes the lifting more difficult. Because it is more difficult, the nervous system fires more and more motor units to stimulate more and more muscle fibers to do the job. Anytime you increase motor unit activation you're likely to increase size and strength as well.
Not too loose: Wrap the bands around the proximal end (near torso) of your limb. Be sure to wrap tight enough that the blood flow return to the torso is restricted. Your veins should bulge and your limb should swell and turn red.
Not too tight: Be sure not to wrap too tight though. You don't want to completely cut blood flow off...just restrict it. If you wrap it too tight, you will cut the blood flow off from entering the limb. This will hurt very bad almost instantly. One tell tale sign is the whitening of the hands and feet. Too tight!
Go light: Use weight that is only 20% of your one repetition maximum. If you can curl 100 lb dumbbells, use only 20's. You will understand why once you try. It hurts.
Sets/Reps/Rest: After reading the research, I chose to go with 4 cluster sets of 30, 15, 15, 15 reps. I rest about 30-45 seconds between each set. Your arms and legs should be screaming by the end of the first set. Be sure to fully relax your limbs between sets and allow blood to swell into the limb. You will know what I'm talking about. At the end of the exercise I leave the bands on as long as I can (usually only a minute or two) for an added stimulus, and also because I hate myself.
Movements: This is really the only time I do isolated movements like curls and triceps extensions. I recommend moving through the entire range of motion (no half reps) to allow for a stretch of the muscle and more blood volume to enter.
Super sets: Occasionally I will super-set opposing muscle groups (bi's/tri's or quads/hammys), to save time.
Add these to the end of your strength training:
Occlusion training did not replace any of my heavy strength training. It's more of an addition to the real hard work I put in. I peform 2-3 sets per week, always at the end of my hard training. It's kind of like the icing on top of the cake. It's a great way to completely waste your muscle in just a few minutes before walking out of the gym. Nothing should replace good old fashion heavy lifting though.
My arms grew 1/4 of an inch in only three weeks. My legs grew as well but I didnt measure so im unsure of how much. Unfortunately, the rapid growth rate decreased drastically after the third week. My tolerance for the pain of this training increased significantly.
I threw up the first time I did occlusion training on legs. I did have them a little too tight though.
What the science says
Don't take my word for it. The science doesn't lie. This training is effective for increasing size AND strength. I've included some links below for your reading pleasure.