"Shouldn't I just go out and hammer as hard as I can on the bike, and then try to hold on during the run?" Well, maybe. Especially during a sprint-distance race. And it is a common strategy, especially for strong cyclists. But devising a strategic plan based on your known power output--and sticking to it--can leave you fresher for the run without slowing you much on the bike. Likely giving you a faster overall time.
The best way for this to work is to have a power meter and know your watts at Functional Threshold (basically, how hard can you pedal for an hour). Knowing how many watts you can hold for 5 minutes and 1 minute is helpful during a hilly race.
On a flat sprint-distance course (12-15 miles), you can probably hold 95-100% of your FT power and still have a good run. On an flat Olympic-distance course, it may only be in the 85-95% range. A good idea is to experiment during your training bricks to see how well you run after riding the race distance at various power outputs.
A hilly course is a bit more complicated. If you've been training with hill intervals of various lengths (1 minute to 20 minutes), you can likely hold a higher wattage on the hills, and use the descents to recover. Climbing hard is one of the best ways to shave minutes off your time.
Below is the step-by-step process I used to plan a strategy for the upcoming Wildlfower Triathon (Olympic-distance course). This will be my first time riding it, so I wanted to do some recon.