A ride followed immediately by a run, with no real break in between. Without bricks you run 10–15% slower than an open run; with regular bricks, only 1–5%.
That leaden feeling in your legs during the first minutes off the bike isn't imagination, it's measurable: running in the transition costs 1.6 to 11.6 percent more energy depending on your level, and the most visible change is an increased forward trunk lean. Brick training trains precisely that switch — and nothing else does. Two mistakes are typical: avoiding bricks because they're unpleasant, and taking a long break between bike and run — then it's two ordinary sessions. Keep the changeover as short as it will be in the race, plan one session a week rather than three at once, and pace the run by heart-rate zone instead of feel — the first minutes feel harder than they are for everyone. What often gets missed: no cadence trick replaces this — the evidence on it is contradictory, and adaptation is the part that's actually supported.