Slip enables you to enter another room and attempt to sneak
up on a target instead of joining in the general fighting.
If the sneaking up is succesful, this gives the chance to
launch an attack that would otherwise be limited to the
start of a fight, like assassinate or blackjack. For other
skills, like vital shot, employing them in this fashion will
increase the chances of success, and the damage done.
If you hesitate for too long after successfully slipping up
on your target, you waste your chance to attack.
Disengage will attempt just that. Instead of joining in the
general mayhem you try to sneak away from combat. Obviously,
this will fail if someone is fighting you directly.
Both skill relies heavily on your ability to sneak and the
alertness of the things going on around you. A high number
of allies fighting improves your chances of succeeding, while
a high number of opponents increases the chances of failure.