Accurate speed control is the key to a comfortable and steady group experience.

Stuff to keep in mind:
The leader-
The group pace is dictated by the pace of the person at the front.
The leader should avoid any sudden or drastic changes in pace. (The most obvious place where this is relevant is to avoid surging when taking the lead)
But unlike the “cruise control” of a car that maintains constant speed, there will be a natural ebb and flow of speed as the terrain and conditions change.

On a descent, and if the conditions allow, the leader should typically pedal to minimize the amount the group, “stacks up” behind.
On an uphill, the leader may want to allow the pace to naturally slow slightly even if it is the type of “roller” that might be sprinted when riding alone.

The group (Every rider is a link in the chain) –
It is especially important for each rider to stay steady at the group pace and not surge or lag.
Every little unnecessary variation becomes magnified as following riders are forced to adjust.

1. So, maintain a constant gap to the rider ahead

2. Instead of watching the gap between wheels, try watching hips (saddle, back, butt, etc.) of the rider ahead to gauge your relative position. It helps  prevent a tendency to “micromanage” the gap and keeps your vision directed ahead rather than down.

3. If and when adjustments are necessary, make them gradually.

4. To slow:
a. “Soft pedal” instead of stopping your pedaling entirely. “Stop and go” pedaling both causes inconsistent speed and is distracting to a following rider. 
b. Gentle slowing can often be accomplished by using the wind, just sitting up a bit or moving just slightly (SLIGHTLY) sideways.  (In conjunction with “a” above)
c. Use brakes as a last resort and gently (feathered) if they must be used at all.

5. To accelerate (close or shorten a gap)
a. As a gap closes the “drafting” benefit becomes more effective, so gradually adjust the effort to avoid harsh actions such as having to hitting the brakes after closing a gap.

High level example (nice visual):
Applicable commentary:
Taken to the extreme:

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