In the recording studio a very popular technique is punch ins. It really just means starting and stopping every few bars. The term originated from the days of tape recording; where the tape record head was literally "punched in". They also had to physically cut and splice the tape for things we do now with copy and paste.

     Punch in freestyle is this same technique applied, instead of reading lyrics it is improvised. You can always keep trying until you get what you like.

     This is a basic layout of a punch scheme. The underlines are implied silence.

This is a post____________
___________about punching in

     When the vocals do not sound like they match, overlay your start and ending points.

This is a post____________
______post about punching in

      Trim "post" off one side and slide one of the blocks vertically so that they are together in one track, apply crossfades where needed.

     During tracking you may not have time to do all that trimming and moving of blocks. A solid alternative is to alternate between two channels. You generally want to avoid creating loads of channels, as this will take longer to process for mixing.