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I have never tried hitting the slide release, with my off hand. Once I insert a new mag, my off hand falls back into a gripping position over my strong hand. It would , for me, be very awkward, to try to hit the slide release with my off hand, because my off hand thumb, falls in behind my shooting hand thumb (if that make sense). It may not be the grip everyone uses, but it works for me.
On my M&P hitting the slide release with my shooting hand thumb, with a full magazine, is very light, and easy to manipulate with my shooting hand thumb. I have seen other posts, where it automatically releases, with the insertion of a full mag, but mine has only done that a couple of times, and It may be because my thumb slightly touched the mag release. If there is some issue where it won't release, I do the slingshot method, to release the slide, but I I have only had to do that once, and it was due to not getting the mag fully inserted, which made me revert to a tap, rack, bang type type scenario, with the slide locked back, not because the slide release, failed to operate properly.
I may look into trying the offhand mag release technique, but just running through it with empty hands feels awkward. JMHO