"and the blade slip from the wood": from the haft (of the axe). Rebbi says: from the wood that was being chopped.
"and it find his neighbor and he die": to exclude his presenting himself. From here R. Eliezer b. Yaakov ruled: If, after the stone left his hand, the other put out his head and was hit by it, the first is not liable.
"he shall flee to one of these cities": He may not exile himself from one city to another.
(Ibid. 6) "Lest the redeemer of blood pursue the slayer": It is a mitzvah for him to pursue him.