Okay, watching over the sense doors, being aware of our sight contact, our hearing contact, taste, touch and so on - that's fine - but if we're trying too hard, headaches will come, you will get stressed out, that's all. So, in this case, we can try to ease up on the effort that we're trying to make, because effort can be too tight.
What happens when the strings are too tight on an instrument? What happens when they're too loose? What happens is pretty obvious for anyone who has ever played a stringed instrument. For those of you who have never done that, it should still be pretty obvious that the sound comes out the wrong way. If it's not obvious for you, play with a rubber band, stretch a rubber band and thump it a bit; you get a different sound all the time.
When a person puts forth too much effort, stress can build. One of the results of stress can be, of course, getting a headache. So, if you get a headache, we want you to acknowledge, "Okay, I must have been trying too hard; I have to ease up on my effort." So you ease up on your effort, and if you ease up too much then you will drift into sloth and torpor, and a similar thing happens, we want you to acknowledge, "Uh-oh, now I'm in sloth and torpor; I guess my strings are too loose." So tighten them up a bit, but not as much as before - and that's exactly how a person tunes a guitar, "Too tight, too loose, too tight, too loose, too tight, too loose, too tight, ah, just right."
So the same thing applies in this practice. If you find yourself getting stressed out, whatever you are doing – then just ease up a bit.