Predicting the outcome of an “opposite side attack” SN2 reaction can be confusing at first, but animations can help. Check out the SN2 animation at chemtube3D.com. To operate the animation, find the drawing of the chemical reaction and click on the forward reaction arrow.
Try to understand the simple reaction from multiple perspectives: 1) the C seems to push its way through its 3 H neighbors to get from leaving group to nucleophile, OR 2) the 3 H neighbors seem to back away from the approaching nucleophile and move to the leaving group’s side of the molecule. You can rotate the animation as it plays so that you can see it from different angles.
Another SN2 animation to watch: HS(-) + (S)-PhCHClCH3