I have this same style grapple, and love it. great for brush and forest debris...takes some style adjustments to pick up logs and rocks. also works great for heavy brush clearing since the top and btm almost touch when closed...just pinch and rip.
to pick up logs and rocks I do this...
open er' way up, and set er' down
move forward until you start to nudge the object
tilt forward...sometimes WAY forward (this causes my skidloader to tip back, can be leveled by raising boom...depends on situation)
close grapple...as you do this roll/tilt back to keep the top tines close to the ground, but not digging in
takes some practice, but becomes second nature...think of it like trying to pick up an M&M off the counter...you want to pinch it
cheers!