Why does it say "Shelach Yadecha"?
Malbim: If You do not want to scatter them via Your Hashgachah Pratis, in any case the salvation should be through Your hand without an intermediary. Do not hide me in a fortress; rather, send Your hand from above and release me from the enemies standing around me; raise me over them.
What is the meaning of "Petzeni"?
Rashi: Take me out. It is an expression of salvation, and similarly "ha'Potzeh Es David Avdo" (10).
Radak: Save me. It is close to "Patzisi Fi" (Shoftim 11:35), which is opening. One who saves a man from a man, he opens him from his incarceration.
What are "Mayim Rabim"?
Radak: They are strong enemies.
Why does it say "mi'Yad Ben Nechar"?
Radak: They are the enemies. After David reigned, his enemies are only Nochrim.