The invoke command is used 
to cast a spell immediately,
or when it is necessary to give
a parameter to the spell.  Invoke
will not set the range weapon.

Examples:
'invoke restoration
'invoke magic rune of large fireball
'invoke reincarnation of Iamdead
'invoke create food of waybread

It is very helpful to bind healing 
spells to keys like this:

'bind invoke medium healing
c

the c key now casts medium healing
instantly