Bike-Run-ICK! That's how your legs feel for the first part of the run. Wobbly. Like Jello-O. It was apparently named by Mark Sisson and Scott Zagarino one day in 1988 after they completed a Bike-Run workout -- "Just another brick in the wall".
I came across an interesting article on Bricks . Doing bricks helped me understand transition, because the practice was useful in gauging myself - how long I took to change gear and how I felt going from one activity to the next. Bricks also actively prepare the muscles for the physical demands of racing. Swim-bike bricks are not as crucial as run-bike bricks, but can give helpful insights. Check out beginnertriathlete.com for some variations on brick workouts.