Please can you briefly explain these techniques in layman's terms to somebody who is yet to experience aerobatics
As noted earlier, an introduction to aerobatics, up to Sportsman level, does not require exceeding -1 to +4G limits even when flying in competition. At +4G the worst you should encounter, even with low blood pressure (I've got that too) is mild symptoms and as the +4G is only sustained for maybe half a second, straining techniques are not really required.
Furthermore, straining techniques are not something you can learn from a book or PPRuNe post. You should practice them with a knowledgeable person present to tell you what you did wrong. Especially since if you apply them in the wrong order or at the wrong moment, you could actually be pushing blood away from your brain instead of pushing towards it.
Having said that, here's what I was taught during that Air Force centrifuge session. It's essentially two things that happen simultaneously. You tense all the muscles in your lower body up to your stomach, and you make sure to keep as much air pressure in your lungs as possible.
Muscles to tense are, from the bottom upwards:
- Toes: Feet flat on the floor, try to curl your toes against the soles of your shoes.
- Lower legs: Push your feet against the floor as if you are extending your feet.
- Upper legs: Try to clench your legs together, at the same time apply a counterforce. Also try to bend your legs, with the floor providing a counterforce.
- Buttocks: clench
- Stomach: clench
At the same time, inhale, block your breath and try to exhale (which you can't). Of course, you need to keep breathing so this is done with quick puffs after which you block/exhale again. (Some people advice you to grunt but really, the sound you make is irrelevant. What's important is to build up the air pressure in your lungs to assist your heart in pumping the blood upwards.)
A very common mistake is to let go of muscle tension as you exhale/puff, which is very dangerous as you can get g-loc very quickly under those circumstances. But you also need to make sure you time the application and amount of these straining techniques properly. If you overdo it, you may actually be more susceptible to g-loc.
The reason for the latter is that in your brain there's a little device (don't know the English word for it) that measures your blood pressure there. If the blood pressure is too low, it tenses the muscles around your arteries. If the blood pressure is too high, it releases the muscles around your arteries. But - and that's an important but - there is a delay of about ten seconds in this reaction.
So if you overdo the straining techniques, or apply them too early, the blood pressure in your brain will rise (since there's no, or not enough g force to counter it). That little device notices this, relaxes the muscles around your arteries to lower the pressure and as soon as the g force comes in, all blood will flow away because you've got no more counterforce to give.
Equally, if you go from, say, minus 3G to an immediate positive, say, 5G, it's extremely dangerous. Even more than going from 1 to 7G, say. This is because under negative G all the blood rushes to your head with rising blood pressure as a consequence. So all your arteries open up. If you then apply positive G all of a sudden, all the blood rushes away.
(The Air Force physician was really surprised to hear that some of the Advanced aerobatics pilots in our group did this. He considered the -3 to +5 situation far more dangerous than the +9 that F16 pilots need to sustain. As there's not a lot of reason to push negative G's in combat, it was something that he had never encountered. They also can't simulate this with their centrifuge.)
Once again, this is only skimming the surface of what's happening in your body and what you can do about it. The lecture we got from this Air Force physician took more than two hours and even after that lecture we did things wrong in the centrifuge - with several g-locs as a result. So please do not assume you can apply the perfect straining techniques just by reading the above text and some practicing in front of the mirror.