The "run" key (by default, this is the shift key) is bound to the
Quake movement command +speed. Pressing this control has
a number of effects: the velocity that a client tries to move at in
any particular direction is doubled; turning speed is increased by the
value of cl_anglespeedkey; and acceleration is also
apparently a little greater than normal.
In the Quake options menu, there is an option called "Always
Run". Intuitively one might expect activating this option to have the
same effect as always holding down the "run" key: in fact, this is not
quite the case. Setting "Always Run" on attempts to simulate the most
noticeable feature of running by doubling the value of
the console variable cl_forwardspeed from its default of
200 to 400, and makes a similar change for
cl_backspeed. However, strafing speed, turning and
acceleration do not appear to be affected. You can check this out by
going forward and strafing simultaneously: you'll see you go at
different angles depending on whether "Always Run" is on or not.
There do not seem to be any advantages gained by using "Always Run"
over simply leaving the "run" control activated, just several minor
disadvantages. Therefore I'd always have advised adding the command
+speed to your config rather than switching on "Always
Run" even before wall-hugging needed to be considered. (Once
optimising wall-hugging becomes a concern, then one may want to
fine-tune the value of client_forwardspeed and so the
"Always Run" option is definitely a no-no.)
A finer degree of control can also be achieved by leaving "Always Run"
switched off. Some good Quake players prefer to do all their running
manually by physically pressing the run key since this allows them to
return to walking speed if they want to do so for more tricky
manoeuvres (like "tight-rope walking" on narrow beams.) A further
alternative is to set +speed on at the start, and then to
turn the "run" key into a "walk" key for when you want to slow down:
alias +walk -speed;
alias -walk +speed;
bind shift +walk;