Those in the highest positions can be vulnerable as many aspire to replace them. Second, if the person is replaced, it’s not easy to find a similar position because of paucity of roles at the top.
If you work only on regular projects, you won’t be challenged and hence not become better at what you do. So, volunteer into few challenging projects as well.
If you’re testing the depth of water with both feet down, you risk being drowned if the water turns out to be deep. But if you test with one foot, you can pull away. The proverb means that before going whole hog into something unknown, assess the risk well.