What it sounds like you're doing isn't exactly engine braking - it's more like compression braking. Or something. When I drove my 'rent's Miata in high school I'd hit 3rd coming off the freeway and quickly let out the clutch - like you're doing from what I've read - and it would actually compression lock the tires. That is, it would prevent the tires from spinning as fast as the car was moving, making them squeal and making the car slow quickly. I didn't do that very often, because it seemed really hard on stuff (2 sets of tires and a clutch in 40k miles?? on a 2000lb car?)