The art and show of weightlifting originated in the Circus. As a display of prowess, uniqueness, strength was a 'freakish' quality reserved for the gawk-worthy fringe. "Look at that man - he can lift two barrels at once!" It wasn't a pursuit worthy of a living, since it wouldn't feed your family, but it was an interesting hobby among many farmboys who dreamed of running away with the circus...
As these lifts became more popular, and modern-day 'shows of strength' became mainstream on beaches and more 'open' cultures around California, competitions were the next obvious step. Around since antiquity in Europe, Olympic Weightlifting began to catch on in North America.
Early on, it became clear to coaches and practitioners that the way to a big lift wasn't strength, but speed: acceleration of the kettlebell, bar, wife... to move the object overhead, fast recruitment of strength was required.The actual mechanism for recruiting force from muscle was not yet understood, but the need for speed was embraced and practiced.
The traditional lifts, though - the press, and the deadlift - were still highly valued as the foundation for speed. The ability to lift 500 pounds off the ground certainly helps a person pull 200 pounds off the ground faster; though the 500lbs wouldn't move as quickly, it was understood that the strength foundation was necessary for the later development of speed. Thus, the 'slow lifts' - which grew to include the front- and back-squat, the deadlift, the press, and the bench press - were built into the sport of weightlifting from the start.
When you're performing a heavy deadlift or back squat, it's immediately obvious to an observer why we call them 'the slow lifts' - your body just isn't moving quickly. What's not obvious, though, is what's happening beneath the surface. Starting from your Central Nervous System, nerves are being activated on the surface of muscle tissue. These nerves activate the muscles in sequence, and at the level necessary to complete the lift, starting with the weakest.
By now, many have heard of the 'slow twitch' muscle fibres and the 'fast twitch' muscle fibres. They're not completely distinct: they exist on a spectrum of fibre in each individual muscle, and tend toward either endurance (slow twitch) or speed (fast twitch.) When lifting a weight, the slow-twitch are first recruited, followed by the fast-twitch, to meet the demand presented by the load. A bigger weight requires the recruitment of more fast-twitch muscle fibre, but the slow-twitch fibre is still recruited first.
The speed at which your body activates these fibres is largely what determines success in weightlifting. We refer to that speed as "Rate of Force Development" around here, and it's also the key to success for other athletes who require speed: MMA fighters, sprinters, powerlifters, wrestlers... Yes, even powerlifters: those brutes with big bellies who certainly don't appear fast.
1) total potential force - your upper strength limit, which in turn can be limited by total muscle fibre;
2) recruitment speed - how quickly the muscle can up-regulate from slow to fast-twitch fibres;
3) recruitment efficiency - how well the CNS or PNS can recruit the right fibres, at the right time.
In classical weightlifting - the sport we call, "Olympic Lifting" for clarity - the bench press, squat, and deadlift were used as assistance lifts to improve strength (#1.) However, they can also be used, at high percentages of your max, to improve #2 and #3. Though the bar may appear to be moving slowly, RFD is rapidly recruiting force across the spectrum of muscle tissue.
Traditionally, when improvement of power is the goal, the base training is done with heavy weight (or light weight for high speed, according to Prilepin's Table) to improve RFD. For the last two months, our programming for CrossFit has included a LOT of the 'slow' lifts as we build our strength base. For members of the Green Army Competitive Team, you'll see this 'strength phase' continue for a little longer, and then taper into the speed-based 'power phase' toward Christmas (and just in time for the SuperMeet.) To reap the benefits of the programming: show up. Do the strength work. This is just a taste of the science behind the writing on the chalkboard.