Often abbreviated to SBM.
A split bottom mount is a common frontstyle mount.
Throw a sleeper. Press your left index finger into the string and up while bringing the right hand down.
... and press your right index finger into the string ...
... and flip the yo-yo into the front string.
Bring your left hand down ...
... mounting the yo-yo in a loop.
This is like a Front Mount over the right hand.