It's called compression lock. The engine is going slower than the drivetrain and the force of compression makes the dirvetrain slow down suddenly, rather than the engine speed up, and the wheels slip because they are going slower than the actual road speed. You need to learn to "heel and toe" to raise the engine revs at downshift which will mean the speeds are equal when you release the clutch. (great for showing off too
). I'm not gonna explain it now so ask around. (just remember when you learn it that it's ALWAYS toe on the brake. NO racing driver does it the other way round [This MAY be my pet hate...]). Have fun.