How is "Notzer Piv Shomer Nafsho"?
R. Yonah: He guards his mouth, not to open it until his heart agrees. This guards his Nefesh!
Malbim: Safah is external speech; Peh is internal speech. Notzer is constantly guarding; this is more than Shomer. The verse depicts that the mouth and lips are a fortified wall around the Nefesh. They enclose it so its enemies cannot approach it. Not only must one guard the lips (outer wall, around which the enemy stands) - external words spoken without Sechel - he must guard greatly also the mouth, which speaks with Chachmah - "Ki ha'Elokim ba'Shamayim v'Atah Al ha'Aretz Al Ken Yihyu Devarecha Me'atim" (Koheles 5:1). This guards his Nefesh - he has a wall around it. Also in Torah and Chachmah one must guard the openings of his mouth.
What is "Posek Sefasav [Mechitah Lo]"?
Rashi: He constantly speaks his entire Ru'ach. Posek is widening, like "va'Tefaski Es Raglayich" (Yechezkel 16:25).
R. Yonah: He accustomed his lips to be in their own Reshus, to speak without prior thought or investigation, or to say all that crosses his Ru'ach. This is a reason for him to fear damage from [what leaves] his lips, just like one fears enemies, for his lips are not in his Reshus.
Malbim: He opens his lips to speak Leshon ha'Ra and Rechilus. This makes also his body open to damage!
