Brick workouts are stacking of two disciplines during the same workout, helping the adaptation between muscle groups. Bricks are a very important part of triathlon training because they help an athlete train like they race. But, if you are new to triathlon you may overlook this part of your training plan if you aren't careful.
As you switch modes of exercise, your body needs to prepare for the next sport while recovering from the previous one. Your heart rate increases significantly as your body tries to shift blood flow from the muscles. Brick workouts help your body handle the demands required of your body through the entire event.
There are basically two type of bricks:
Swim/bike brick
Bike/run brick
In the swim/bike brick, you want to begin using your legs as little as possible in the swim so you can have an easier time getting on your bike and feeling comfort in your legs. Having a workout that simulates race conditions helps you minimize this problem. During a brick workout, kicking harder in the last part of the swim or keeping your bike in an easier gear than normal will help your legs get accustomed to what they will feel like on race day.
In the bike/run brick your legs will literally feel like...bricks...when you come off the bike. This type of workout will not only allow you to get use to that "brick" feeling but the more you do it the better you will feel and the faster you will run. It's not uncommon for people to experience cramps after biking so you will sometimes need to listen to your body and slow down if you feel a cramp is coming. Simulating race-day conditions, you may want to ride a medium distance followed by a run of medium distance.
One important part of a brick is to keep moving just like you were in a race. Set up your transition area, practice a smooth, fast transition, and continue on to the next sport.
Happy bricking!