The McNasty is defined as at least 150° horizontal angle rotation or half a barrel roll flowing into a front loop or space godzilla.
Basically it is half a spin flowing into a loop.
Start in a back surf and start to initiate a spin so your nose starts moving towards a front surf. As this is happening, load up the nose of your boat by pushing down on your feet. The idea is to use the top side of the nose of your boat to be used to push the nose down into the downwards moving water which will in effect pop your boat up and out of the water.
You can start by practicing the move on the flatwater. Start by edging your boat and complete a sweep stroke, pushing your nose into the water. This will feel weird as you are used to edging your boat towards the other direction, especially for a spin.
Another way of completing a Mcnasty is you can turn your boat back in the direction that you started your spin. This is commonly called a switch McNasty. These are good in pour over holes. To help get your boat more height, look up in the air and reach up with your arms. A common mistake is to try to throw your McNasty too early which will shank your move preventing the loop from coming around straight.
Now to get some extra air out of your moves try to use a slight backstroke before you move into your push stroke, this will get your boat further upstream and into a back blast position. This will allow the bow of your boat down and into and the fastest moving part of the hole. Remember to look up and bring your arms up into the air, this will also help you get bigger air in your moves.
Now get out there and practice your McNasty, practice makes perfect and be prepared to fail numerous times before you get it.
Cheers mate, enjoy this free piece of instruction.