There are a bunch of psionic feats that require you to either maintain (easy) or expend your psionic focus. Regaining your psionic focus is normally a standard action which provokes opportunity (although there are other feats that can help with that)
Example psionic feats include
PSIONIC WEAPON [PSIONIC]
You can charge your melee weapon with additional damage potential.
Prerequisite: Str 13.
Benefit: To use this feat, you must expend your psionic focus.
Your attack with a melee weapon deals an extra 2d6 points of damage. You must decide whether or not to use this feat prior to making an attack. If your attack misses, you still expend your psionic focus.
SPEED OF THOUGHT [PSIONIC]
The energy of your mind energizes the alacrity of your body.
Prerequisite: Wis 13.
Benefit: As long as you are psionically focused and not wearing heavy armor, you gain an insight bonus to your speed of 10 feet.
UNAVOIDABLE STRIKE [PSIONIC]
You can make an unarmed strike or use a natural weapon against your foe as if delivering a touch attack.
Prerequisite: Str 13, Psionic Fist, base attack bonus +5.
Benefit: To use this feat, you must expend your psionic focus. You can resolve your unarmed strike or attack with a natural weapon as a touch attack.
You must decide whether or not to use this feat prior to making an attack. If your attack misses, you still expend your psionic focus.
MENTAL LEAP [PSIONIC]
You can make amazing jumps.
Prerequisite: Str 13, Jump 5 ranks.
Benefit: To use this feat, you must expend your psionic focus. You gain a +10 bonus on a Jump check.
INQUISITOR [PSIONIC]
You know when others lie.
Prerequisite: Wis 13.
Benefit: To use this feat, you must expend your psionic focus.
You gain a +10 bonus on a Sense Motive check to oppose a Bluff check.
You must decide whether or not to use this feat prior to making a Sense Motive check. If your check fails, or if the opponent isn’t lying, you still expend your psionic focus.