To understand what a hover tunnel and why this is so tricky to avoid, read this excellent article published at UXMovement : Why Hover menus do more harm than good.
Amazon.com's homepage uses these menus extensively yet remains easy to navigate. The engineers at Amazon do this by predicting the direction that your mouse pointer is most likely to take when you start hovering. Based on where your cursor is, one can draw a triangle that connects the cursor to the two ends of the menu (shown below). Imagine your cursor is at the green point (right above "Amazon Cloud Drive"). Now as long as your mouse cursor is inside the red triangle, the "Amazon Cloud Drive" menu will remain open even if your mouse hovers above "Appstore for Android" or "Digital Games & Software".