Engine breaking

Let's fix the spelling first - it's engine braking and if everything goes well it should not break the engine

It is the engine working in the reverse way - when you close the throttle at high speed, the engine stops making enough power to sustain or increase the speed, so the rear wheel begins to 'push' the engine and finds resistance in the compression that the pistons have to overcome in the cylinders.

This resistance slows the bike down. In the 4-strokes engine the spark plug ignites the fuel half of the times it does in a 2-strokes, so in this situation the engine's compression to be overcome is higher and the bike slows down much more rapidly - in fact, often too much.

Slipping the clutch decreases the amount of power transferred from the rear wheel to the pistons, i.e. decreases the engine braking - just as in the active phase, slipping the clutch decreases the power transferred from the pistons to the rear wheel.

the easiest way to explain it is, try pushing a vehicle while its in gear without the clutch being pulled in, it the engines compression that causes this braking effect.

In laymens terms...i call it clutch play.

But here ya go...

Engine braking is the act of using the energy-requiring compression stroke of the internal combustion engine to dissipate energy and slow down a vehicle. Compression braking is a common legal term for the same mechanism. Large trucks use a device called an exhaust brake to increase the effectiveness of engine braking.

Compression of gas and vapor requires energy as described by theories in physical chemistry and thermodynamics. Compression in an engine is driven by the forward momentum of the vehicle as well as the angular momentum of the flywheel. When a driver downshifts to spin the engine at high angular velocity (or RPM) without pressing on the [accelerator pedal], the engine converts energy from the vehicle's speed, which is kinetic energy, into a temperature increase in the fuel-air mixture. These hot gases are exhausted from the vehicle and heat is transferred from engine components to the air.

This energy conversion occurs because most four stroke internal combustion engines require compression of the fuel-air mixture before ignition, in order to extract useful mechanical energy from the expansion. Diesel engines are adiabatic and have no spark plugs and use energy transferred to air charge during compression to directly ignite the mixture when the fuel is injected.

The advantage of using the engine to dissipate energy is this immediate ejection of energy. Hot gases are ejected from the vehicle very quickly and the gases also transfer much of their heat directly to engine parts. In addition, friction produced within the engine system also adds heat to the engine parts.

This engine heat is taken away by the engine's integrated cooling system: usually a liquid circulation system and a radiator. Disc or drum brakes have no such energy dissipation mechanisms. They must rely on air flow to remove heat and they retain heat without producing temperatures that would deform and damage the brakes.

Placing a vehicle in a low gear causes the engine to have more leverage (mechanical advantage) on the road and the road to have less leverage on the engine. This is what allows cars to slow down using their relatively flimsy engine parts. The engine maintains a high rotational speed to dissipate a lot of power without forcing too much strain on the engine.

The exhaust brake is used in large diesel vehicles because the rate of conversion of mechanical energy into waste thermal energy is low compared to the mechanical returns to kinetic energy from the air-spring effect in the engine.