What's the better braking strategy for steep, technical descents?
- "Brake hard, then release completely & roll."
- "Maintain constant pressure on the brakes."
Aaron's answer is great but since you mention the words "downhill" and "technical descents" the rule of thumb is that "constant pressure on the brakes" is not appropriate. It does not allow you to ride safely, enjoy the ride and evolve as a rider. That's because:
On the other hand, not braking everywhere on the course will let you go over obstacles, roots, rocks easier. That's especially important for the front wheel which, even when under minimal braking, when it hits an obstacle it doesn't react very well. It transmits a lot of feedback on the handlebars and unless you know what you are doing may give you slight problems (regarding riding stance, line choice, balance) which is something that is not welcome when riding downhill.
Note though that riding downhill is hard. If you find yourself unable to do proper braking on a course then maybe it's too steep for you. Find a flatter course and try to master all of its parts with minimal and purposeful braking.
Short answer: depends.
Braking is as much a style as other parts of riding, but there are some general guidelines. The longer you hold those brakes, the more the heat will build up and braking performance will decrease. You'll also burn through pads a lot faster that way. On the other hand, sudden braking puts quite a bit of stress on the bike all the way from the rotor through the fork/rear triangle, to the handlebars and chain. It also changes the way the suspension moves through it's travel.
When should use either of these methods? I generally use a longer, controlled brake when descending a steep chute that requires a bit more technical riding or doesn't have a great runout after the rough bits. Quick braking is useful when you're plowing into a corner and need to scrub some speed quick or plan on drifting around it. You can also brake right before dropping in to a section so that the wheels roll over the rocks and roots instead of sliding and then roll out the bottom.
Really it's going to come down to how you ride and where you like to make speed changes. Experiment and find what's comfortable and what meets your needs. If you're trying to carry crazy speed you might find a quick, hard brake is more beneficial, etc.