LINE 55:
RX=INT(X/256)	RX is the integer of X/256 which means that RX is rounded off to 0 when X is less than 256, and RX becomes 1 when X reaches position 256. We will use RX in a moment to POKE V+16 with a 0 or 1 to turn on the "RIGHT SIDE" of the screen.
LX=X-RX*256	When the sprite is at X position 0, the formula looks like this: LX = 0 - (0 times 256) or 0. When the sprite is at X position 1 the formula looks like this: LX = 1 - (0 times 256) or 1. When the sprite is at X position 256 the formula looks like this: LX = 256 - (1 times 256) or 0 which resets X back to 0 which must be done when you start over on the RIGHT SIDE of the screen (POKEV+16,1).
LINE 60:
POKEV,LX	You POKE V by itself with a value to set the Horizontal (X) Position of sprite 0 on the screen. (See SPRITEMAKING CHART on Page 176). As shown above, the value of LX, which is the horizontal position of the sprite, changes from 0 to 255 and when it reaches 255 it automatically resets back to zero because of the LX equation set up in line 55.
POKEV+16,RX	POKEV+16 always turns on the "right side" of the screen beyond position 256, and resets the horizontal positioning coordinates to zero. RX is either a 0 or a 1 based on the position of the sprite as determined by the RX formula in line 55.
LINE 70:
IFP=192THEN GOSUB200	If the sprite pointer is set to 192 (the first sprite shape) the waveform control for the first sound effect is set to 129 and 128 per line 200.