There are two key elements in your 2 stroke motor.
1. Cranking compression
2. Crank Case compression
Cranking compression is that which is measured thru the spark plug hole when the piston hits TDC (Top Dead Center).
Crank Case compression is that which is measured at the reed cage when the piston hits BDC (Bottom Dead Center). Sometimes you see crank descriptions as saying they are "stuffers". This is a term that describes a crank that has a larger volume of metal or material to take up space in the crank case, thus reducing crank case volume and increasing crank case compression.
When you space the reeds outward from the motor, you essentially increase the volume of air space in the crank case, thru decreasing the crank case compression ratio. This can be beneficial if you have a motor that will turn upwards of 12+k rpms and has a very short power band. Not something that too many Blaster can do.
Placing the spacer on the back side of the reeds does a couple of things as well.
First of course it spaces your carb reward away from the reeds and adding clearance from your clutch arm (if you need it).
Second, placing more mixed air/fuel at the base of the reeds is great for a short lil burst, but does slow down velocity thru the carb and reduces emulsification. It tends to cool the motor better, since your motor will be receiving more "puddled" fuel but decreases total efficiency and energy created by the combustion process due to the "unmixed" fuel being burnt.
Take a look at EFI systems. When first introduced, they sprayed fuel just below the throttle plate. Today they have a much more fine mist sprayed directly at the cylinder head intake port. The closer you can get your carb to the motor, the better it will perform.
Hope this help explain the process and effect a lil better. It is by no means the end of all that takes place in a 2 stroke motor.
I would caution anyone that buys a "stuffer" crank before you talk to a good engine builder that can measure your crank case compression ratio first. Over stuffing the crank can actually hurt you if you dont have the transfer ports that can support the additional boost of air, reeds/intake that can deliver the additional mixture that will be needed. If your motor is not designed or modified to handle this extra flow the motor will bind much the same as if you had too much cranking compression. Balancing these two is the key to your motors success.
Any motor builder that cannot tell you these compression ratios in your motor is probably just copying someone else's design that has worked for others and not really done the homework or tested their changes over time. There are lots of copycats out there.
Ask your builder this and see what they tell ya.