He used that technique to escape from the sealing . But later on he is unable to use some techniques. What happen during the sealing?
The technique used by Lord Mū was the Fission Technique.
This technique allows Mū to literally split his own body into two identical yet entirely independent duplicates of himself.
Though it might seem to be as same as the Clone Technique ,
the ability is instead a physical separation and so does not require hand seals to activate, making it faster in comparison as a result. This also means that even if one is incapacitated, the other remains completely unaffected and as neither can truly be regarded as the original, the duplicates cannot be dispelled like normal clones.
As a result of being split into to two entities ,
However, while split, Mū's power is halved and as such he is unable to use more complex techniques like his Dust Release kekkei tōta.
Later on :
Unable to remove the seal so he could reunite with his other half, Mū was made to begin summoning "that" reincarnation. Due to his weakened state, it took longer for Mū to complete the summoning.
"that" here refers to Madara Uchiha.
Mu used a technique during the Tsuchikage's attack, where he split his body into 2. This technique cause his power to be split into 2 as well, causing him unable to cast jutsu afterwards. The one that Kazekage sealed was one half of him, while the other half ran away and later summoned Uchiha Madara.