If you want to conserve some of the short-term variation in the data within the 'linearized' area, select the 'Conserve variation' option.   This uses a 3-point nearest-neighbor smoothing operation to reduce the amount of deflection within the defined area, but it preserves the point-to-point variation.   You can select how much linearizing will occur with the pop-up menu of smoothing repetitions ('smooth once' = 1 smoothing pass, i.e., the most variance preseved, and 'smooth X5' is 5 smoothing passes, resulting in the greatest degree of linearizing).   As more smoothing passes are added, the corrected data increasingly resemble a straight line between the beginning and ending points (this is best visualized by experimenting with the three options, using the 'delete' key after each one to eliminate the correction).

You can alternate between spike removal mode and linearizing mode by first clicking in the lower left window, then clicking the desired mode button, and finally returning to the plot area.  Accessing the 'Quit' button is done similarly.

If you are in multi-channel display mode, the program will shift to single-channel display (of the currently active channel) for spike removal operations.  If the file contains more than screenwidth cases and you are in Entire File display mode, the program will shift to Active Screen mode when interactive spike removal is selected.  Use the screen slider to shift between screenfulls of data (note that the data linearizing option can work across screen boundaries).

Some automatic spike-fixing functions are available with the Trim window.