weakening my grip has cured my slice?

Had a lesson today - I was convinced before I got there that I was swinging OTT Instructor said no way - if anything its inside out. He said my grip was causing lack of release - I asked if I should try and roll hands over earlier - he said that's a no-no and that release is the effect of good grip pressure, swing path and grip being neutral. It felt horrible going to neutral grip, if anything I felt like I had metal rods in my arms. I couldn't argue with the results though, straight or draw, at least 10 yards longer and all of a sudden my swing path/divots are square. How the hell can a weaker grip cure a slice and why does a neutral grip feel so strange? Almost feels like I am not in control but the balls are flying arrow straight? Thanks

Btw I had a strong grip before

I've had a version of this problem before.  An overly strong grip basically encouraged me to close off my hips and shoulder line too much on the back swing in an unconscious effort to sort of try to square my body with my grip, setting my body and swing plane up for a big push.  Then many things can happen.  If you stay from way inside on the down swing you can utilize the strong grip to force the club face to get square to the aim line, causing a nasty hook.  Sometimes you just stay on that push line and hit a straight push.  But sometimes for me at least I'd also basically yank my front side out halfway through the down swing in an attempt to get back to my target line instead of the push line.  If your hands stay on target to remain square to the push line you set up in the back swing but you yank across to get your swing line square to the target line, then you've got an open face and a square swing plane at impact, and you're going to hit a slice to push slice.

I'm sure there's other explanations that more knowledgeable players might be able to give you, but that's one way I've gotten a slice out of a strong grip.  This was especially true for me with the driver or 3w off the tee, where the ball is up and forward, giving the yank a few more inches to sort of wrap what started as an inside-out swing plane into an out-to-in path at impact.

I recently changed from a strong grip to a neutral grip with great results. Going with a strong grip to cure a slice seems to be joining a long list of terrible advice instructors have been giving out forever.

so does this mean my instructor sounds like he knows what he is talking about?

Your grip has a lot to do with the point at which your wrists release, with your very strong grip you were probably releaseing the wrists after contact leading to an open face through the ball and a slice looking trajectory. Neutral grip releases the wrist a bit earlier and now you hit the ball square. Magic the hard part is knowing weather the grip is part of the problem and weather to tighten or loosen the grip which it seems your instructor has been able to help you with.

can someone tell me why a neutral grip feels so out of control and lacks power?

but the opposite is true?

Youre just not used to it. Any other grip for me feels weird.

Just sit around and hold a club in neutral grip.  You'll think other grips are weird after you get used to it

I got the same advice from my instructor.  I suffered from a big arc to the left on my irons.  I bit the bullet and did a mid-season grip change from strong with right and left thumbs almost at 3 o'clock to a neutral one.  It felt strange and awkward but it took only about 2 trips to the driving range to see results.  With a better grip came more confidence in my swing.  More confidence in my swing seem lets me swing harder and faster for more accuracy and distance.  Now I need to relax the death-grip I have in my left hand and I think I will be good to go.

does grip have an effect on swingpath/plane?

Swing plane is not the only way to make a slice.