This extension lets you sample 3 feet deeper than the length of your sampling tube. (6 feet with a 3 foot tube) If you want to sample deeper than that you will need to purchase one or more JMC ESP Plus Regular Extensions in addition to this extension.
Assembly fits manual sliding hammer for the JMC Environmentalist's Sub-Soil Probe PLUS (1.2 inch diameter sampler). This is the part that rests on the top of the sampling tube that fits up inside the heavy sleeve of the hammer.