As UMHH mentioned, IWT needs to be pressed each time your character(s) need to move or turn towards an enemy. A lot of people (myself included) just end up sending our IWT keys with our DPS spells, so it gets spammed during combat. This can lead to other issues, like toons running off after the mob dies, or them spinning eradically around the target. For both issues you need a bind for backpeddling to stop them. Ideally you could just use a separate bind for your IWT for more control, but atleast I found that I fights with a lot of movement I ended up just spamming that, so it was just easier to send it with the DPS keys.

As far as the issue of your lead dying: Most people use ISBoxer nowadays, which sets up an FTL (Focusless, Targetless, Leadersless) setup for you automatically. The system works so your Assist and Follow keys send different commands to your other toons, based on the character you are currently leading from, making it possible to switch your lead character on the fly and not have it change the functionality of your team one bit. This is perfectly achievable with other software solutions as well, but does take a bit more work.